BAGHDAD, Aug 10 (Reuters) - Islamic State militants have killed at least 500 members of Iraq's Yazidi minority, burying some alive and taking hundreds of women as slaves, an Iraqi government minister told Reuters on Sunday.

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 a "a vicious atrocity" with cheers and weapons waved in the air. No independent confirmation was available.

Islamic State's advance through northern Iraq has forced tens of thousands to flee, threatened the capital of the Kurdish autonomous region and provoked the first U.S. air strikes in the area since Washington withdrew troops from Iraq in late 2011.

Sudani said in a telephone interview that news of the killings had come from people who had escaped town of Sinjar, an ancient home of the Yazidis, a Kurdish-speaking community whose religion has set them apart from Muslims and other 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."

President Barack Obama said on Saturday that U.S. air strikes had destroyed arms that the Islamic State, which has captured swathes of northern Iraq since June, could have used against the Iraqi Kurds. However, he warned 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 organization, we are fighting a terrorist state."

In comments likely to put pressure on Washington to step up its response to Islamic State, Iraqi rights minister 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 have collected on the desert top of nearby Mount Sinjar, seeking shelter from the insurgents.

At the Vatican, Pope Francis held a silent prayer for victims of the Iraqi conflict, who include members of the Christian minority, during his weekly address on Sunday.

"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.

"All of this deeply offends God and deeply offends humanity."

MALIKI CRITICISM

Speaking before U.S. warplanes struck militant targets for a second day on Saturday, Obama said it would take more than bombs to restore stability, and criticized Prime Minister Nuri al-Maliki's Shi'ite-led government for failing to share power with Iraq's Sunni minority, which dominated before the U.S. invasion.

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, and all Iraqis should feel that they are represented in this government," Foreign Minister Fabius said.

"All Iraqis should feel they are represented to take part in this battle against terrorism," he told a news conference with his Iraqi counterpart in Baghdad.

Maliki's critics say his sectarian agenda has prompted heavily armed Sunni tribes to join the insurgency. But Maliki, serving in a caretaker capacity since an inconclusive election in April, has defied calls by Sunnis, Kurds, fellow Shi'ites, regional power broker Iran and Iraq's top cleric to step aside for a less divisive leader.

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 a civil war peaked in 2006-2007.

Nearly every day police report kidnappings, bombings and execution-style killings in many cities, towns and villages.

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.

A former head of German intelligence echoed the Kurdish plea for arms: "The Kurds are hopelessly outgunned," August Hanning told the Bild am Sonntag newspaper.

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 routed Kurdish forces and 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; Writing by Michael Georgy; Editing by David Stamp and Alastair Macdonald)