(Corrects first name of U.S. envoy in 16th para to Keith)
* Iraqi minister warns of global threat
* Reports of atrocities like nightmares - U.S. envoy
* Session expected to authorise investigation team
* Russia, Syria chide West for letting militants thrive
By Stephanie Nebehay
GENEVA, Sept 1 (Reuters) - Islamist fighters have carried out atrocities on "an unimaginable scale" in months of fighting with Iraqi forces who have also killed detainees and shelled civilian areas, a U.N. official said in an emergency debate on the conflict on Monday.
Iraq's human rights minister, Mohammed Shia' Al Sudani, told the session that Islamic State militants, "oozing with barbarity", threatened his country and the world, but did not immediately respond to allegations against state troops.
Islamic State has grabbed large areas of Iraq and neighbouring Syria, declaring a cross-border caliphate and driving hundreds of thousands from their homes. At least 1,420 people were killed in Iraq in August alone, U.N. figures showed on Monday.
The one-day session, called by Iraq with the support of allies including the United States, is expected to agree to Baghdad's request to send a team of U.N. experts to investigate crimes committed in the conflict.
There is "strong evidence" Islamic State and allied groups have carried out targeted killings, forced conversions, sexual abuse and torture in Iraq, U.N. Deputy High Commissioner for Human Rights Flavia Pansieri said, opening the debate in Geneva.
"The reports we have received reveal acts of inhumanity on an unimaginable scale," she told the U.N. Human Rights Council, on its first meeting about the latest surge in violence. She later told Reuters she was referring to Islamic State.
Iraqi government forces and police had also committed acts that may amount to war crimes, Pansieri said.
"Systematic and intentional attacks on civilians may constitute war crimes and crimes against humanity. Individuals, including commanders, are responsible for these acts," Pansieri said, referring to crimes committed by both sides.
"BURNED ALIVE, BEHEADED"
Pansieri said she was particularly worried about the persecution of Christians, Yazidis, Shia, Turkmen and other ethnic groups by Islamic State forces that have swept through western and northern Iraq.
"These communities have lived side by side, on the same soil, for centuries and in some cases for millennia," she added. Such "ethnic and religious cleansing" may amount to crimes against humanity, Pansieri added.
Iraqi police have also executed detainees in Tal Afar and government-allied militias opened fire on a mosque in Khanaqin district northeast of Baghdad killing 73 men and boys, she said.
Iraqi soldiers have shelled towns and carried out air strikes near Kirkuk, Falluja, and Salahuddin, killing and injuring many dozens of civilians, she added.
Iraq's human rights minister Mohammed Shia' Al Sudani told the session Islamic State was threatening the makeup of his country.
"The land of ancient Babylon is subjected to threats starting to its very independence, they are attempting to change its demographic and cultural composition," he said in Arabic.
Islamic State was not just a problem for his country, he added. "It is a trans-national phenomenon that poses an imminent danger to all countries of the world, it defies all human rights principles and international law."
The U.S. ambassador to the rights forum, Keith Harper, urged Iraq's Prime Minister designate Haider al-Abadi to set up a multi-ethnic government that would investigate all allegations against government forces and "terrorist groups".
"The stories that have emerged from ISIL's (Islamic State's) bloody assault on Iraq are the ones of nightmares. Christians and others have been driven from their homes with the threat of 'convert or die'," Harper said.
"The Yazidis have been buried alive, beheaded or killed in mass executions," he said.
Western and Gulf countries denounced Islamic State abuses. "This organisation has nothing to do with Islam, even if they carry the name," said Kuwait's ambassador Jamal Al-Ghunaim.
But Russia and its ally Syria blamed Western and Arab states for allowing the Sunni extremists to thrive in the region.
Russia had supplied arms to counter Islamic State, including Sukhoi 25 strike aircraft, Moscow's ambassador Alexey Borodavkin said.
"ISIL acquired a huge potential: it now controls colossal financial resources that it has seized, is pursuing illegal oil trade and has a considerable arsenal of modern weaponry. All this could have been avoided if the international community had taken measures at the time to remove this cancer at an early stage of its formation," he said. (Additional reporting by Tom Miles; Editing by Andrew Heavens)