The militant offensive has cleared the way for Iraqi Kurds - who were once gassed by former dictator Saddam Hussein - to take control of a swath of disputed territory they want to incorporate into their autonomous region over Baghdad's strong objections.

Crucially, their security forces are now responsible for securing the ethnically mixed oil-rich city of Kirkuk, the heart of that disputed territory.

"Now we are living a different era," Barzani told CNN, as he called for Prime Minister Nuori al-Maliki, whom he described as "the one responsible for what has happened" in Iraq, to step down.

"During the last 10 years we did everything in our ability... to build a new democratic Iraq, but unfortunately the experience has not been successful."

Pressed on whether Iraqi Kurds would seek independence, Barzani said: "The time is here for the Kurdistan people to determine their future and the decision of the people is what we are going to uphold."

"Iraq is obviously falling apart anyway, and it's obvious that a federal or central government has lost control over everything."

US officials acknowledged that the gains made by Kurds in recent weeks in the fighting might not be easily reversed.

At the same time, Washington is keen to persuade the Kurds to speed up the formation of a new government following April elections.

Under a de facto system in Iraq following recent elections, a Kurd has traditionally held the presidency, a Shia Arab has been the prime minister and a Sunni Arab has been the speaker of parliament.

"The Kurds have a better role to play in maintaining the stability of the state here to some extent," a senior State Department official told reporters.

"If they decide to withdraw from the Baghdad political process, it will accelerate a lot of the negative trends," he warned.

But if the Kurds take an active role "they will have substantial clout and influence in Baghdad".

The region has also taken in thousands of people displaced by the militant offensive, and Kerry's visit aimed to recognise their efforts to tackle "a very serious refugee crisis".

