U.S. Secretary of State John Kerry held crisis talks with leaders of Iraq's autonomous Kurdish region on Tuesday urging them to stand with Baghdad in the face of a Sunni insurgent onslaught that threatens to dismember the country.
Iraqi security forces fought Sunni armed factions for control of the country's biggest oil refinery 200 km (120 miles) north of Baghdad, under threat for nearly two weeks since militants overran northern cities.
Kerry flew to the Kurdish region after a day in Baghdad on an emergency trip through the Middle East to rescue Iraq after a lightning advance by Sunni fighters led by an al Qaeda offshoot, the Islamic State in Iraq and the Levant.
U.S. officials believe that persuading the Kurds to stick with the political process in Baghdad is vital to keeping Iraq from splitting apart.
"If they decide to withdraw from the Baghdad political process it will accelerate a lot of the negative trends," said a senior State Department official who briefed reporters on condition of anonymity.
Kurdish leaders have made clear that the settlement keeping Iraq together as a state is now in jeopardy.
"We are facing a new reality and a new Iraq," Kurdish President Massoud Barzani said at the start of his meeting with Kerry. Earlier, he blamed Prime Minister Nuri al-Maliki's "wrong policies" for the violence and called for him to quit, saying it was "very difficult" to imagine Iraq staying together.
The 5 million Kurds, who have ruled themselves within Iraq in relative peace since the U.S. invasion that toppled Saddam Hussein in 2003, have seized on this month's chaos to expand their own territory, taking control of rich oil deposits.
Two days after the Sunni fighters launched their uprising by seizing the north's biggest city Mosul, Kurdish troops took full control of Kirkuk, a city they consider their historic capital and which was abandoned by the fleeing Iraqi army.
The Kurds' capture of Kirkuk, just outside the boundary of their autonomous zone, eliminates their main incentive to remain part of Iraq: its oil deposits could generate more revenue than the Kurds now receive from Baghdad as part of the settlement that has kept them from declaring independence.
Some senior Kurdish officials suggest in private they are no longer committed to Iraq and are biding their time for an opportunity to seek independence. In an interview with CNN, Barzani repeated a threat to hold a referendum on independence, saying it was time for Kurds to decide their own fate.
Washington has placed its hopes in forming a new, more inclusive government in Baghdad that would undermine the insurgency. Kerry aims to convince Kurdish leaders to sign on.
Read MoreKerry in Cairo for talks over Iraq