US Secretary of State John Kerry has vowed “intense and sustained support” for Iraq after meeting key politicians in the capital, Baghdad.
His visit comes as Sunni insurgents expand their control of towns across the country’s north and west.
The rebels are bearing down on a vital dam near Haditha, and have captured all of the border crossings to Syria and Jordan from government forces.
The key airport in the northern town of Tal Afar has also fallen to the rebels.
There are reports the town itself has also fallen. Tal Afar controls the main road from the Syrian border to Mosul, Iraq’s second biggest city, which was captured by the rebels two weeks ago.
Heavily armed Iraqi troops are protecting the dam near Haditha. Residents told BBC Arabic that rebels had surrounded the town but had not yet entered it.
An Iraqi military spokesman said that hundreds of Iraqi soldiers had been killed by Sunni Arab militants in the current offensive.
On Monday Mr Kerry met Iraqi Prime Minister Nouri Maliki to discuss the crisis. Mr Kerry also met key Shia and Sunni figures.
The secretary of state later said: “The support will be intense, sustained, and if Iraq’s leaders take the steps needed to bring the country together it will be effective.”
A statement from Mr Maliki’s office said the crisis in Iraq represented “a threat not only to Iraq but to regional and international peace”.
Iraqi officials briefed on the meeting told the Associated Press that Mr Maliki had asked for targeted US air strikes, but that Mr Kerry had expressed concern about civilian casualties.
The BBC’s Richard Galpin in Baghdad says Mr Kerry will have been pressing for a new government, more inclusive of minority Sunni Arabs.
Officials said the rebels of the Islamic State in Iraq and the Levant took two key crossings in Anbar on Sunday, a day after seizing one at Qaim, a town in the province that borders Syria.
The capture of Tal Afar airport is a blow to the government, which was hoping to use it as a springboard to recapture Mosul, says the BBC’s Jim Muir in northern Iraq.
Police sources also told the BBC that 70 prisoners had been killed near the city of Hillah, south of Baghdad.
The prisoners, who were all accused of terrorism, were being moved further south for security reasons when the convoy came under attack by gunmen.
They were killed in the crossfire. Several policemen were injured and six of the gunmen were shot dead, police said.
Analysis by Jim Muir, BBC News, in Irbil
John Kerry is trying to persuade politicians across the board to rise above the sectarian and ethnic divides and come together to pull their country back from the brink of fragmentation.
The question is no longer whether Iraq is splitting up – it is. The question is whether that process can somehow be reversed. The odds are not good.
A future Iraq will clearly have to involve a large measure of devolution, if not actual partition. It could happen bloodily, or by agreement.
It is unlikely Mr Kerry will find a single Iraqi leader apart from Prime Minister Maliki himself who believes the incumbent premier is the man to lead a reconciliation process necessary for a political solution.
But if Iran insists Mr Maliki has to stay – as it has with Bashar al-Assad in Syria – the chances of a settlement will be sharply reduced.
A solution would require some kind of understanding between the two major outside players, the US and Iran, but there is little sign of a meeting of minds so far.
Since the fall of Mosul in early June, Isis has helped win large areas in the west and north.
It has taken four strategically important towns in predominantly Sunni Anbar province – Qaim, Rutba, Rawa and Anah – in the past two days.
Gunmen have captured the border posts of Walid, on the Syrian frontier, and Traybil, on the Jordanian border, after government forces pulled out.
The capture of frontier crossings could help Isis transport weapons and other equipment to different battlefields, analysts say.
The US, which pulled out of Iraq in 2011, is deploying some 300 military advisers to Iraq to help in the fight against the insurgents.
But Iran’s Supreme Leader Ayatollah Ali Khamenei said he opposed any US intervention, and accused Washington of “seeking an Iraq under its hegemony and ruled by its stooges”.
“The main dispute in Iraq is between those who want Iraq to join the US camp and those who seek an independent Iraq,” he said, dismissing talk of sectarianism.