Meanwhile, in New York, the United Nations Security Council urged world powers "to support the government and the people of Iraq and to do all it can to help alleviate the suffering of the population."
Iraqi Ambassador Ali al-Hakim said the meeting focused on the need for urgent relief efforts to help civilians fleeing the violence, but denied reports that air strikes had been carried out against the jihadists.
"There is no strike being done yet," he said. "The first item is immediate humanitarian help for Iraq, inside of Iraq. That is immediately requested and it looks like it's being done right now."
In northern Iraq, Holgar Hekkmat -- a spokesman for the peshmerga, the forces of Iraq's autonomous Kurdish region -- said US F-16s had entered Iraqi airspace to target IS units in Gwer and the Sinjar region.
The Pentagon firmly denied this, but the White House denounced the IS offensive, warned of a "humanitarian catastrophe" and warned that US President Barack Obama was examining his options.
"There are times where the president has taken military action ... to protect innocent, vulnerable civilian populations from slaughter," spokesman Josh Earnest said, recalling NATO's 2011 Libya campaign.
- Mountain siege -
Earnest told White House reporters: "There is a mountain near Sinjar where there are reports that thousands of Yazidis are currently trapped on that mountain and have been for a couple of days now.
"They are unable to access food and water. They don't have any access to shelter. They have fled persecution, and efforts to leave the mountain are blocked by ISIL forces who are vowing to kill them."
A Pentagon official said: "We have been working urgently and directly with officials in Baghdad and Arbil to coordinate Iraqi airdrops to people in need."
Separately, French President Francois Hollande's office said "France was available to support forces engaged in this battle.""
Obama came to office determined to end US military involvement in Iraq and in his first term oversaw the withdrawal of the huge ground force deployed there since the 2003 American invasion.
But recent rapid gains by the Islamic State, a successor group to Al-Qaeda's former Iraqi and Syrian operations, compelled him to send military advisors back to Baghdad to evaluate the situation.
The group, along with allied Sunni factions, is at war with Iraqi Prime Minister Nuri al-Maliki's mainly Shiite government forces and with the peshmerga forces of the Kurdish autonomous region of the country.
In late June it proclaimed a "caliphate" straddling rebel-held areas of Syria and Iraq and seized the major city of Mosul. In recent days it has seized towns formerly populated by Christians and Yazidis.
- 100,000 Christians flee -
Iraqi religious leaders say Islamic State militants have forced 100,000 Iraqi Christians to flee and have occupied churches, removing crosses and destroying manuscripts.
"Qaraqosh, Tal Kayf, Bartella and Karamlesh have been emptied of their original population and are now under the control of the militants," Joseph Thomas, the Chaldean Catholic archbishop of Kirkuk and Sulaimaniyah, told AFP.
Entirely Christian Qaraqosh lies between Mosul, the jihadists' main hub in Iraq, and Arbil, the Kurdish region's capital. It usually has a population of around 50,000.
Tal Kayf, the home of a significant Christian community as well as members of the Shabak Shiite minority, also emptied overnight.
"I heard some gunshots last night and, when I looked outside, I saw a military convoy from the Islamic State... shouting 'Allahu Akbar'," said Boutros Sargon, a resident who fled and was reached by phone in Arbil.
Meanwhile, several thousand Yazidis, members of an ancient pre-Muslim religious minority, are stranded on high ground after being driven out of their home town of Sinjar by IS fighters.
Fares Sinjari Abu Ivan, a Yazidi beekeeper who fled with his 80-year-old mother to the barren mountains, told AFP by phone that some groups had attempted to flee but experienced mixed fortunes.
"We have spoken to some who made it to Turkey but in their flight, they encountered Daash (Islamic State) fighters who cut the road. Some fled, some were killed and others came back to the mountain."
Turkish officials said up to 800 displaced Sinjaris had made their own way to Turkey, while the PKK Kurdish separatist group said it had evacuated several families after opening a safe passage to Syria.
The Islamic State boasted of its latest victories, declaring: "We are pleased to announce to the Islamic nation a new liberation in Nineveh province, teaching the secular Kurds a lesson."