Iraq has formally called on the US to launch air strikes against jihadist militants who have seized several key cities over the past week.
"We have a request from the Iraqi government for air power," confirmed top US military commander Gen Martin Dempsey in front of US senators.
Earlier the Sunni insurgents launched an attack on Iraq's biggest oil refinery at Baiji north of Baghdad.
Iraqi PM Nouri Maliki earlier urged Iraqis to unite against the militants.
Government forces are battling to push back ISIS (Islamic State in Iraq and the Levant) and its Sunni Muslim allies in Diyala and Salahuddin provinces, after the militants overran the second city, Mosul, last week.
US President Barack Obama met senior Congress members on Wednesday to discuss the Iraq crisis. The White House said Mr Obama had "reviewed our efforts to strengthen the capacity of Iraq's security forces to confront the threat from ISIL [ISIS], including options for increased security assistance".
Ahead of the briefing Senate leader Harry Reid, a Democrat, said he did not "support in any way" getting American troops involved in the Iraqi "civil war".
But Gen Dempsey told a Senate panel that it was in America's "national interest to counter [ISIS] wherever we find them".
In other developments:
- UK Prime Minister David Cameron told Parliament in London that ISIS was also plotting terror attacks on Britain
- India confirmed that 40 of its citizens had been kidnapped in the violence-hit Iraqi city of Mosul
- Saudi Foreign Minister Saud bin Faisal warned that Iraq faced the risk of civil war
- Turkey is investigating reports that 15 Turkish builders were abducted by ISIS on Tuesday; 80 Turks were kidnapped in Mosul last week
Paul Adams, World Affairs correspondent
A detailed look at what ISIS says about itself, in two annual reports, is very revealing.
Get past the gruesome audit of violence - the numbers of people they claim to have killed through car bombs, suicide attacks and even "apostates run over" - and a picture emerges of an "increasingly structured organisation", in the words of an analysis by the Institute for the Study of War (ISW).
The statistics show a major ISIS focus, over the past two years, on Nineveh province, which may help to explain the Iraqi army's headlong flight from Mosul last week. More than 30% of ISIS attacks in both 2012 and 2013 were focussed on Nineveh, with a particular emphasis on threats against members of the Iraqi military and intimidation of local journalists.
But the reports suggest ISIS has nationwide ambitions, to take over large parts of the country. In the absence of a considered strategy, warn the authors of the ISW study, ISIS "will become a permanent fixture in the Middle East".
Militants 'in control'
Earlier on Wednesday, the Iraqi military said it had driven off ISIS fighters attacking the Baiji refinery, 210km (130 miles) north of Baghdad.
The attack reportedly started at 04:00 (01:00 GMT) from outside two of the three main entrances to the refinery.
Army spokesman Qasim Ata said in news conference broadcast live on TV: "The security forces thwarted an attempt by ISIS to attack the Baiji refinery and 40 terrorists were killed."
But an unnamed official told the Reuters news agency the rebels were in control of 75% of the refinery.
The nearby town of Baiji was overrun by ISIS-led militants last week. Foreign personnel, including a small number of British nationals, were evacuated from the refinery earlier but local staff reportedly remained in place.
Hundreds of people have been killed since the start of the militant offensive, many of them believed to be captured soldiers publicly shot by ISIS-led firing squads.
Analysis: Richard Galpin, BBC News, Baghdad
The refinery which is now shut down is critical for Iraq's supplies of petrol and other petroleum products. It supplies a quarter of the country's refining capacity. The power plant is equally important, as the country still suffers from acute shortages of electricity.
It was always likely the militants would target Baiji. The area is dominated by Sunni Muslims and was a hotbed of insurgent activity during the US occupation, making it fertile ground for ISIS.
And ISIS has a track-record of seizing lucrative assets such as refineries.
In an interview with the BBC, the representative of the Shia spiritual leader, Grand Ayatollah Ali al-Sistani, said Iraq's - religious leaders had sensed "a real danger which threatens Iraq and its unity".
Speaking to world affairs editor John Simpson in his first ever interview, Sheikh Abdul Mahdi Karbalai said the response should be a "a stance which is adopted by people from all sects".
'Mercenaries and terrorists'
Prime Minister Maliki described the ISIS onslaught as a setback, but added: "Not every setback is a defeat."
"This setback has allowed Iraq to recover its national unity," he said on Wednesday.
Mr Maliki has long been accused of favouring the country's Shia Muslim majority and fomenting unrest among the Sunni minority.
Iranian President Hassan Rouhani said Tehran will not "spare any effort" to defend Shia holy shrines in Iraq against "mercenaries, murderers and terrorists".
He was speaking amid reports that the head of the elite Quds Force of the Iranian Revolutionary Guards, Qasem Soleimani, was in Baghdad to help co-ordinate the fight against the militants.
Some 500,000 people have been internally displaced since the capture of Mosul, according to UN estimates.
ISIS in Iraq
ISIS grew out of an al-Qaeda-linked organisation in Iraq
- Estimated 10,000 fighters in Iraq and Syria
- Joined in its offensives by other Sunni militant groups, including Saddam-era officers and soldiers, and disaffected Sunni tribal fighters
- Exploits standoff between Iraqi government and the minority Sunni Arab community, which complains that Shia Prime Minister Nouri Maliki is monopolising power
- ISIS led by Abu Bakr al-Baghdadi, an obscure figure regarded as a battlefield commander and tactician