(Adds car bombs, last paragraph)
* Hundreds of prisoners executed by Islamic State, says U.N.
* Syria ready to cooperate in fight against Islamic State
* Iranian official in visit to Saudi Arabia
* Islamic State claims mosque revenge attack
By Stephanie Nebehay and Ahmed Rasheed
GENEVA/BAGHDAD, Aug 25 (Reuters) - The United Nations on Monday condemned "appalling, widespread" crimes by Islamic State forces in Iraq, including mass executions of prisoners that could amount to war crimes.
U.N. human rights chief Navi Pillay condemned "grave, horrific human rights violations" being committed by Islamic State, a Sunni Muslim group which has seized large areas of Iraq and Syria to the alarm of the Baghdad government and its allies in the West.
Up to 670 prisoners from Badush prison in the city of Mosul were killed by Islamic State on June 10, Pillay said in a statement quoting survivors and witnesses to the "massacre" as telling U.N. human rights investigators.
"Such cold-blooded, systematic and intentional killings of civilians, after singling them out for their religious affiliation, may amount to war crimes and crimes against humanity," Pillay said.
Islamic State (ISIL) loaded 1,000 to 1,500 prisoners from the jail on to trucks and took them for screening, Pillay said. Sunni inmates were then separated and removed.
"ISIL gunmen then yelled insults at the remaining prisoners, lined them up in four rows, ordered them to kneel and opened fire," she said.
Islamic State fighters have made gains against Kurdish forces in the north in recent weeks, seizing towns, oilfields and Iraq's largest dam. Backed by U.S. air power, Kurdish forces later took back control of the Mosul dam.
An Islamic State video last week depicting the beheading of American journalist James Foley prompted revulsion in the West and calls for tougher action against the jihadists, including taking the fight to them in Syria as well as Iraq.
Some experts have suggested that attacking Islamic State in Syria should involve coming to some sort of arrangement with the government of President Bashar al-Assad, seen in the West as a pariah since an uprising against him began three years ago.
Syria said it would cooperate in any international efforts to fight Islamic State in the country, after Washington signaled it was considering extending the battle against the militants into Syrian territory.
Foreign Minister Walid al-Moualem presented his country as a vital partner in the war against Islamic State.
"Syria, geographically and operationally, is the center of the international coalition to fight Islamic State," Moualem said in a televised news conference. "States must come to it if they are serious in combating terrorism," he added.
Asked about the prospect of U.S. air strikes against Islamic State in Syria, Moualem said his government was ready to cooperate with any country fighting militants. But air raids without Damascus's approval would be seen as hostile acts.
While the White House indicated last week that it was considering taking on Islamic State in Syria, Washington has also supported the insurgency against Assad and there has been no sign of any shift in U.S. policy toward him.
GERMANY KEEPS DISTANCE
Germany said on Monday it has had no diplomatic contacts with the Assad government and no plans to rekindle ties because of the threat posed by Islamic State.
The statement by a German foreign ministry spokesman followed a report in The Independent, a British newspaper, which said the United States had shared intelligence with Syria via Germany's BND intelligence service.
"The regime of President Assad has committed unbelievable injustice in every form during the civil war that has been raging for 3-1/2 years. Nearly 200,000 people have died," the spokesman, Martin Schaefer, told a news conference.
"To be honest it is very difficult to imagine that all this can be ignored in the name of Realpolitik," he said.
Russia, Syria's major ally, urged Western and Arab governments to overcome their distaste for Assad and engage with him to fight Islamic State insurgents. "I think Western politicians are already realizing the growing and fast-spreading threat of terrorism," Foreign Minister Sergei Lavrov said.
The growing perception in the West and in Baghdad that Islamic State represents a threat to the region and beyond has shaken old alliances and enmities.
While there have been suggestions that the West may find itself dealing with Assad, old enemies Iran and Saudi Arabia have united in welcoming this month's appointment of incoming Shi'ite Prime Minister Haider al-Abadi in Iraq.
Iranian Deputy Foreign Minister Hossein Amir Abdollahian left for Riyadh on Monday, the state news agency IRNA reported. This would mark the first visit to Saudi Arabia by a senior government official since President Hassan Rouhani was elected in 2013, promising to try to improve Tehran's relations in the region and with the West.
Shi'ite Muslim Iran and Sunni Saudi Arabia are enmeshed in a struggle for influence in the Middle East and back opposing sides in conflicts and political disputes in Iraq, Syria, Lebanon, Bahrain and Yemen.
IRNA said Abdollahian was due to meet Saudi Foreign Minister Prince Saud Al Faisal. Riyadh officials were not available to comment, but Saudi-owned satellite news channel al-Arabiya said the Iranian minister would arrive on Tuesday for talks.
The visit follows talks in Baghdad on Sunday between Abadi and Iranian Foreign Minister Mohammad Javad Zarif, who reaffirmed Tehran's support for Iraq's territorial unity and its fight against militants.
Abadi said on Monday that talks on forming a new government were constructive and predicted a "clear vision" on a unified administration would emerge within the next two days, state television reported.
Abadi is tasked with forming a power-sharing government that can tackle deepening sectarian violence and counter Islamic State.
In Baghdad, a suicide bomb attack in a Shi'ite mosque on Monday killed at least nine people and wounded 21, police and medical sources said.
Islamic State claimed responsibility for the bombing, saying in a statement that it was to avenge an attack on Friday when Shi'ite militiamen opened fire in a Sunni mosque in Diyala province north of Baghdad on Friday, killing 68 people.
Later on Monday, two car bombs killed at least 11 people and wounded 25 in a mainly Shi'ite district of Baghdad, police and medical sources said. One exploded in a busy street and another at a restaurant in the Utaifiya district in the west-central area of the capital. (Additional reporting by Michael Georgy and Kareem Raheem in Baghdad, Mariam Karouny in Beirut, Katya Golubkova in Moscow, Noah Barkin in Berlin, Michelle Moghtader in Dubia and Angus McDowall in Riyadh; Writing by Giles Elgood; Editing by David Stamp and Jonathan Oatis)