* Iraqi minister warns of global threat
* Reports of atrocities like nightmares - U.S. envoy
* Session expected to call for investigation team (Adds speeches by Iraq, United States, quotes, background)
By Stephanie Nebehay
GENEVA, Sept 1 (Reuters) - Both Islamist fighters and, to a lesser extent, Iraqi government forces have killed civilians and committed atrocities in three months of fighting, a senior 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 fighters, "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 of Iraqis from their homes. At least 1,420 people were killed in Iraq in August alone, U.N. figures showed. (Full Story)
There was "strong evidence" Islamic State and allied movements had carried out targeted killings, forced conversions, abductions, sexual abuse and torture, 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.
Iraqi government forces had also committed acts that may amount to war crimes, Pansieri said - though she later told Reuters they were not on the scale of the atrocities carried out by Islamic State.
"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.
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.
"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, Stephen 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.
(Reporting by Stephanie Nebehay; Editing by Andrew Heavens)