US 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.

Security forces fought Sunni armed factions for control of the country's biggest oil refinery on Tuesday and militants launched an attack on one of its largest air bases less than 100 kilometers from the capital.

More than 1,000 people, mainly civilians, have been killed in less than three weeks, the UN said Tuesday, calling the figure "very much a minimum."

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 Syria (ISIS).

US officials believe that persuading the Kurds to stick with the political process in Baghdad is vital to keep 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 US 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 Nouri 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 US 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.

The Kurds' capture of Kirkuk 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 join it.

Baghdad is racing against time as the insurgents consolidate their grip on Sunni provinces.

The Baiji refinery, an industrial complex 200 kilometers north of Baghdad, remained a frontline early on Tuesday. Militants said late on Monday they had seized it, but two government officials said troop reinforcements had been flown into the compound and fended off the assault.

Local tribal leaders said they were negotiating with both the government and Sunni fighters to allow the tribes to run the plant if Iraqi forces withdraw. One of the government officials said Baghdad wanted the tribes to break with ISIS and other Sunni armed factions, and help defend the compound.

The plant has been fought over since last Wednesday, with sudden reversals for both sides and no clear winner so far.

In the town of Yathrib, 90 kilometers north of Baghdad, tribes aided by ISIS fighters attacked the huge al Bakr air base with mortars, according to a security source and the deputy head of the municipality.

In recent days, Baghdad's grip on the western frontier with Syria and Jordan has been challenged. One post on the Syrian border has fallen to Sunni militants and another has been taken over by the Kurds. A third crossing with Syria and the only crossing with Jordan are contested, with anti-government fighters and Baghdad both claiming control.