The president of the Kurdish region of northern Iraq has issued a defiant statement to the Iraqi government that there was no going back on autonomous Kurdish rule in the oil city Kirkuk.

Massoud Barzani, president of the Kurdistan Regional Government, made the comments at a joint news conference in Erbil with visiting William Hague, British foreign secretary, on Friday.

"We waited for 10 years for Baghdad to solve Article 140," he said, referring to the constitutional item which was meant to address the Kurds' decades-old ambition to incorporate the territory in their autonomous region in the north over the objections of successive governments in Baghdad.

"Now its accomplished because the Iraqi army pulled out and our Peshmerga forces had to step in. So now the problem is solved. There will be more no more conversation about it."

Al Jazeera's Hoda Abdel-Hamid, reporting from Erbil, said Barzani's statement was expected to put more strain on the Baghdad government.

"The Kurds see themselves in a position of strength, and say the Iraqi government's pullout forced Peshmerga forces to fill the security vacuum," she said.

Kurdish forces stepped in when federal government forces withdrew in the face of a Sunni rebel offensive led by the Islamic State of Iraq and the Levant (ISIL) earlier this month.

The Sunni rebels made the gains as Iraq's flagging security forces were swept aside by the initial insurgent push, pulling out of a swathe of ethnically divided areas.

The Iraqi army carried out airstrikes on Tikrit, and launched an assault on a strategic university campus on Friday to recapture the rebel-held city.

Exclusive video obtained by Al Jazeera showed damage inside the city after reports of Iraqi military helicopters flying commandos into the city on Thursday.

Several locals told Al Jazeera there were no rebels in the area and that the military hit targets indiscriminately.

Nouri al-Maliki, who has been Iraq's prime minister since 2006, has faced intense pressure to form an inclusive government and address the longstanding grievances of the Sunni and Kurdish communities.

Sistani urges unity

On Friday Ayatollah Ali al-Sistani, Iraq's leading Shia religious leader, became the latest prominent figure to distance himself from Maliki when he called on politicians to unite and choose a prime minister before parliament sits next week to begin forming a government.

Sistani, who commands unswerving loyalty from many Shia in the region, said the various political blocs should agree on the next prime minister, parliament speaker and president before the newly elected legislature meets on Tuesday.

Under Iraq's governing system put in place after the fall of Saddam Hussein, the prime minister has always been a Shia, the largely ceremonial president a Kurd and the speaker of parliament a Sunni.

Dividing up the three posts before parliament meets would require leaders from each of Iraq's three main ethnic and sectarian groups to commit to the political process and resolve their most pressing problems, including Maliki's fate.

"What is required of the political blocs is to agree on the three [posts] within the remaining days to this date," Abdul Mehdi Karbalai, Sistani's spokesman, said during a Friday prayer sermon in the Shia shrine city of Karbala.

Maliki, whose Shia-led State of Law coalition won the most seats in the April election, had been positioning himself for a third term before the onslaught began.

Despite the turmoil and calls both domestically and internationally for him to step down, Maliki has said any attempt to undermine him would be tantamount to a "coup".