* Minister says some victims buried alive, women kidnapped
* Militants ordered Yazidis to convert or die
* U.S. conducts third day of air strikes
* France echoes U.S. call for inclusive Iraqi government
* Maliki blamed for worst crisis since Saddam's fall (Adds Maliki comments)
By Ahmed Rasheed
BAGHDAD, Aug 10 (Reuters) - Islamic State militants have killed hundreds of Iraq's minority Yazidis, burying some alive and taking women as slaves, an Iraqi government minister said on Sunday, as U.S. warplanes again bombed the insurgents and a political deadlock dragged on.
Human rights minister Mohammed Shia al-Sudani accused the Sunni Muslim insurgents - who have ordered the community they regard as "devil worshippers" to convert to Islam or die - of celebrating what he called a "a vicious atrocity".
No independent confirmation was available of the killings of hundreds of Yazidis, an event that could increase pressure on Western powers to do more to help tens of thousands of people, including many from religious and ethnic minorities, who have fled the Islamic State's offensive.
The U.S. Central Command said drones and jet aircraft had hit Islamic State armed trucks and mortar positions near Arbil, the capital of the autonomous Kurdish region which had been relatively stable throughout the past decade of turmoil until the insurgents swept across northwestern Iraq this summer.
That marked a third successive day of U.S. air strikes, and Central Command said in its statement that they were aimed at protecting Kurdish peshmerga forces as they face off against the militants near Arbil, the site of a U.S. consulate and a U.S.-Iraqi joint military operations centre.
President Barack Obama has urged Iraqi political leaders to bury their sectarian differences and form a more inclusive government that can unite Iraqis against Islamic State militants.
Prime Minister Nuri al-Maliki, seen as a sectarian leader, was in no mood for compromise, indicating in a tough speech delivered on television on Sunday night that he would not drop his bid for a third term.
Special forces loyal to him were deployed on Sunday night in strategic areas of Baghdad, police said. Several police sources also said the forces had taken up positions at key entrances to the sprawling capital.
Maliki, seen as an authoritarian and sectarian leader, accused Iraqi Kurdish President Fouad Masoum of violating the constitution by failing to meet a deadline for asking Iraq's biggest political bloc to nominate a prime minister.
A dispute over which bloc won the most seats during the election has complicated efforts to form a new government in Iraq, a major oil exporter.
Maliki, who has served in a caretaker capacity since an inconclusive election in April, has defied calls by Sunnis, Kurds, some fellow Shi'ites and regional power broker Iran to step aside for a less polarising figure who can unite Iraqis against Islamic State militants.
"I will submit today an official complaint to the federal court against the president of the Republic for committing a clear constitutional violation for the sake of political calculations," said Maliki.
The U.S. State Department said on Sunday it had pulled some of its staff from the Arbil consulate for their safety.
The Islamists' advance in the past week has forced tens of thousands to flee, threatened Arbil and provoked the first U.S. attacks since Washington withdrew troops from Iraq in late 2011, nearly nine years after invading to oust Saddam Hussein.
Iraqi rights minister Sudani told Reuters in a telephone interview that accounts of the killings had come from people who had escaped the town of Sinjar, an ancient home of the Yazidis, a Kurdish-speaking community whose religion has set them apart from Muslims and other local faiths.
"We have striking evidence obtained from Yazidis fleeing Sinjar and some who escaped death, and also crime scene images that show indisputably that the gangs of the Islamic State have executed at least 500 Yazidis after seizing Sinjar," he said.
"Some of the victims, including women and children were buried alive in scattered mass graves in and around Sinjar."
Consolidating a territorial grip that includes tracts of Syrian desert and stretches toward Baghdad, the Islamic State's local and foreign fighters have swept into areas where non-Sunni groups live. While they persecute non-believers in their path, that does not seem to be the main motive for their latest push.
The group wants to establish religious rule in a caliphate straddling Syria and Iraq and has tapped into widespread anger among Iraq's Sunnis at a democratic system dominated by the Shi'ite Muslim majority following the U.S. invasion of 2003.
Obama warned on Saturday that there was no quick fix for the crisis that threatens to tear Iraq apart.
Kurdish regional president Masoud Barzani urged his allies to send arms to help his forces hold off the militants, who have bases across the Syrian border. During a visit by French Foreign Minister Laurent Fabius, Barzani said: "We are not fighting a terrorist organisation, we are fighting a terrorist state."
Another senior Kurdish official said Kurds retook two towns southwest of Arbil, Guwair and Makhmur, with the help of U.S. strikes. But he did not expect a rapid end to the fighting.
In Jalawla, 115 km (70 miles) northeast of Baghdad, a suicide bomber killed 10 Kurdish forces and wounded 80 people on Sunday. Kurdish fighters and Islamic militants are locked in fierce clashes in the town.
Fabius, noting how Islamic State fighters had taken the upper hand after seizing heavy weaponry from Iraqi troops who fled in June, said the European Union would look into bolstering the Kurds' arsenal to help them hold out and hit back.
TAKEN AS "SLAVES"
Sudani said: "The terrorist Islamic State has also taken at least 300 Yazidi women as slaves and locked some of them inside a police station in Sinjar and transferred others to the town of Tal Afar. We are afraid they will take them outside the country.
"In some of the images we have obtained there are lines of dead Yazidis who have been shot in the head while the Islamic State fighters cheer and wave their weapons over the corpses," he added. "This is a vicious atrocity."
A deadline passed at midday on Sunday for 300 families from the Yazidi community - followers of a religion influenced by the Zoroastrianism of ancient Persia - to convert to Islam or die. It was not immediately clear if the victims to whom the minister referred were from that group of families.
U.S. military aircraft have dropped relief supplies to tens of thousands of Yazidis who are trapped on the desert top of nearby Mount Sinjar, seeking shelter from the insurgents.
A spokesman for the U.N. refugee agency in Iraq said some 30,000 Iraqis had since Friday reached safety in Kurdistan after travelling on the Syrian side of the border from Sinjar.
At the Vatican, Pope Francis held a silent prayer for victims of the Iraq conflict: "Thousands of people, among them many Christians, banished brutally from their houses, children dying of hunger and thirst as they flee, women kidnapped, people massacred, violence of all kinds," he said in a Sunday address.
"All of this deeply offends God and deeply offends humanity."
Obama said it would take more than bombs to restore stability, and criticised Maliki's Shi'ite-led government for failing to share power with Iraq's Sunni minority, which was dominant under Saddam.
Critics say Maliki, an unknown when he first took office in 2006 with help from the United States, has neglected Sunnis while placing his Shi'ite political loyalists in key government and military positions.
France joined the calls for Iraq's feuding leaders to form an inclusive government capable of countering the militants.
"Iraq is in need of a broad unity government," Foreign Minister Fabius said in Baghdad. "All Iraqis should feel they are represented to take part in this battle against terrorism."
The pressure from France came a day after Obama described the upheaval in the north as a "wake-up call" to Iraqis who have slipped back into sectarian bloodshed not seen since 2006-2007.
Nearly every day police report kidnappings, bombings and execution-style killings in many cities, towns and villages. In Baghdad, police were on Sunday manning some squares in armoured personnel carriers, an unusual sight.
The Islamic State, which sees Shi'ites as infidels who deserve to be killed, has met little resistance. Thousands of U.S.-trained Iraqi soldiers fled when its Arab and foreign fighters swept through northern Iraq from eastern Syria in June.
The collapse of the Iraqi army prompted Kurds and Shi'ite militias to step in, with limited success.
The Sunni militants routed Kurds in their latest advance with tanks, artillery, mortars and vehicles seized from fleeing Iraqi troops, calling into question the Kurds' reputation as fearsome warriors.
Iranian-trained Shi'ite militias may stand a better chance than the Kurds but they are accused of kidnapping and killing Sunnis, playing into the hands of the Islamic State, which also controls a large chunk of western Iraq.
After hammering Kurdish forces last week, the militants are just 30 minutes' drive from Arbil, the Iraqi Kurdish capital, which until now has been spared the sectarian bloodshed that has scarred other parts of Iraq for a decade.
The possibility of an attack on Arbil has prompted foreigners working for oil companies to leave the city and Kurds to stock up on AK-47 assault rifles at the arms bazaar.
In their latest sweep through the north, the Sunni insurgents seized a fifth oil field, several more villages and the biggest dam in Iraq - which could give them the ability to flood cities or cut off water and power supplies - hoisting their black flags up along the way.
After spending more than $2 trillion on its war in Iraq and losing thousands of soldiers, the United States must now find ways to tackle a group that is even more hardline than al-Qaeda and has threatened to march on Baghdad. (Addiornal reporting by Isla Binnie in Rome and Missy Ryan in Washington; Writing by Michael Georgy and David Stamp; Editing by Alastair Macdonald, Sonya Hepinstall and Sandra Maler)