The United States warned Thursday a jihadist offensive in northern Iraq could provoke a "humanitarian catastrophe," amid reports that President Barack Obama was considering US military action.

A Pentagon official told AFP the US military is already helping the Baghdad government coordinate air drops of humanitarian aid to civilians and the White House did not rule out future direct action.

"It is a situation that we are looking at very closely," White House spokesman Josh Earnest said, following reports that Obama was talking with military advisors about options for intervention.

Earnest would not confirm the reports that US air strikes are on the table, but said American personnel were studying conditions on the ground in cooperation with Iraqi security forces.

"So if there are specific needs that need to be met in terms of enhancing Iraq security forces' capabilities, then we will look to provide it," he added, without giving further details.

He would not be drawn on the likelihood of strikes, but compared the situation to that in Libya in 2011, when US jets joined NATO allies in a bombing campaign sold as heading off a massacre of civilians.

"There are times where the president has taken military action ... to protect innocent, vulnerable civilian populations from slaughter or other dire humanitarian situations," Earnest said.

"There is one particular situation that we are concerned about."

- Mountain siege -

"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. And they are -- 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 Erbil to coordinate Iraqi airdrops to people in need."

Erbil is the seat of the Kurdish regional government.

US media, citing senior White House officials, said Obama was weighing military options for strikes against the jihadists and aid drops to the displaced and besieged civilians.

Asked about the reports, Earnest said: "I'm not in a position to rule things on the table or off the table in this context."

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 (IS), 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 UN Security Council was to hold emergency talks on the crisis later Thursday, and France has pledged support for forces "engaged in battle" against the IS radicals.

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.

Iraqi religious leaders say Islamic State militants have forced 100,000 Iraqi Christians to flee and have occupied churches, removing crosses and destroying manuscripts.

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.