President Barack Obama ordered US warplanes back into the skies over Iraq on Thursday to drop food to refugees and if necessary launch air strikes to halt what he said was a potential "genocide."
The US air armada's first mission was to drop food and water to thousands of members of the Yazidi religious minority besieged by Sunni extremist fighters from the so-called Islamic State (IS). Tens of thousands of Iraqi Christians have also fled.
Obama warned that he had also authorised the military to carry out targeted strikes in support of Iraqi forces to break the Islamists' advance or to protect US advisors working on the ground.
The president said US warplanes could also target Islamic State militants if they advance on the city of Arbil, where the US has a diplomatic presence and advisors to Iraqi forces.
"We plan to stand vigilant and take action if they threaten our facilities anywhere in Iraq, including the consulate in Arbil and embassy in Baghdad," he said.
A senior US defence official confirmed the mission had already dropped "critical meals and water for thousands of Iraqi citizens," referring to Yazidis trapped in the open on Mount Sinjar in northern Iraq. There will be more aid drops if necessary, another US official told reporters.
Obama said there were perhaps tens of thousands of civilian refugees, and he accused the IS of attempting "the systematic destruction of the entire people, which would constitute genocide."
The president admitted the United States can not act every time it sees injustice, but insisted: "We can act, carefully and responsibly to prevent a potential act of genocide.
"That's what we're doing on that mountain. I therefore authorized targeted air strikes if necessary to help forces in Iraq as they fight to break the siege and protect the civilians trapped there," he added.
Despite this note of determination, Obama was at pains to assure war-weary Americans that he -- the president who withdrew US forces from Iraq -- was not about to get "dragged into fighting another war."
"American combat troops will not be returning to fight in Iraq because there is no American military solution to the larger crisis in Iraq," he said.
- 'Wake-up call' -
Earlier, 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. 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 deteriorating situation.
The Sunni extremists, along with other Sunni factions, are 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, IS proclaimed a "caliphate" straddling rebel-held areas of Syria and Iraq and seized the major city of Mosul. In recent days, it has taken over towns formerly populated by Christians and Yazidis.
Iraqi religious leaders say the militants have forced 100,000 Iraqi Christians to flee and have occupied churches, removing crosses and destroying manuscripts.
Entirely Christian Qaraqosh was among the towns taken over by militants, they told AFP.
US Secretary of State John Kerry, in a strongly worded statement, accused IS of waging a "campaign of terror against the innocent, including Yazidi and Christian minorities, and its grotesque and targeted acts of violence bear all the warning signs and hallmarks of genocide.
"For anyone who needed a wake-up call, this is it."
Republican senators John McCain and Lindsey Graham however accused Obama of not going far enough. "We need a strategic approach, not just a humanitarian one. A policy of containment will not work against ISIS (IS)," they said in a statement.
- Stranded -
Those who had fled described the militant onslaught.
"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 refugee who was reached by phone in Arbil.
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 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."