Sunni militants have seized the northern Iraqi city of Tal Afar, coming closer to the capital Baghdad, raising hopes of an unusual regional security cooperation between the US and Iran, with possible support from Syria.
Militants led by ISIS - the Islamic State in Iraq and the Levant - captured key Iraqi cities, including Mosul and Tikrit, last week, although the Iraqi army managed to retake some towns, possibly with Syrian help.
Syria's army has been pounding major bases of the Islamic State of Iraq and Greater Syria in coordination with Baghdad for the last 24 hours, an activist group said on Sunday.
The strikes against ISIS - which has spearheaded a week-long jihadist offensive in Iraq - have been more intense than ever, said the Syrian Observatory for Human Rights.
"The regime's air force has been pounding ISIS's bases, including those in the northern province of Raqa and Hasakeh in the northeast," which borders Iraq, said the Britain-based group.
The unprecedented conflict and reports of "major human rights violations", including summary executions of civilians, could lead to a broader coordination by the US, Iran and Syria in the fight against the invading militants.
The US says it may use drone strikes to halt the militants' advance. But that may not help much to halt their march on Baghdad.
"They're not the whole answer, but they may well be one of the options that are important," said US secretary of state John Kerry.
Iraq's prime minister Nouri Maliki is a close ally of Iran and Washington, which is frustrated with Maliki's Shia sectarianism, may want Tehran to persuaded Maliki to be more inclusive in his politics.
"When you have people murdering, you have to stop that. And you do what you need to do if you need to try to stop it from the air or otherwise," Kerry said, adding that his country was also open to holding direct talks with Iran over Iraq.
Iranian President Hassan Rouhani earlier said he would consider co-operation if the US took action.
The USS George HW Bush aircraft carrier is already being deployed to the Gulf, accompanied by two more warships. But Washington says no US troops will be deployed on the ground.
Meanwhile, fighting is raging in Tal Afar, with mortar shelling of some districts as militants try to enter the city. The city has a mixed Sunni and Shia population and some ethnic Turkmen.
The Iraqi government insists the town is still largely in government hands with the commanding general there saying he expected to finish off the ISIS fighters within a matter of hours, according to a BBC report.
The government also says it has "regained the initiative" against an offensive by Sunni rebels.