Iraqi forces launched an airborne assault on rebel-held Tikrit on Thursday with commandos flown into a stadium in helicopters, at least one of which crashed after taking fire from insurgents who have seized northern cities.

Witnesses said battles were raging in the city, hometown of former dictator Saddam Hussein, which fell to Sunni Islamist fighters two weeks ago on the third day of a lightning offensive that has given them control of most majority Sunni regions.

The helicopters were shot at as they flew low over the city and landed in a stadium at the city's university, a security source at the scene said. Government spokesmen did not respond to requests for comment and by evening the assault was still not being reported on state media.

The source, speaking on condition of anonymity, said fierce clashes ensued, centred around the university compound.

Ahmed al-Jubbour, professor at the university's college of agriculture, described fighting in the colleges of agriculture and sports education after three helicopters arrived.

"I saw one of the helicopters land opposite the university with my own eyes and I saw clashes between dozens of militants and government forces," he said.

Jubbour said one helicopter crash-landed in the stadium. Another left after dropping off troops and a third remained on the ground. Army snipers were positioning themselves on tall buildings in the university complex.

Iraq's million-strong army, trained and equipped by the United States, largely evaporated in the north after Sunni fighters led by the Islamic State in Iraq and the Levant launched their assault with the capture of the north's biggest city Mosul on June 10.

But in recent days, government forces have been fighting back, relying on elite commandos flown in by helicopter to defend the country's biggest oil refinery at Baiji.

A successful operation to recapture territory inside Tikrit would deliver the most serious blow yet against an insurgency which for most of the past two weeks has seemed all but unstoppable in the Sunni heartland north and west of Baghdad.

Maliki under pressure

In the capital, the president's office confirmed that a new parliament elected two months ago would meet on Tuesday, the deadline demanded by the constitution, to begin the process of forming a government.

Prime Minister Nuri al-Maliki, whose Shi'ite-led State of Law coalition won the most seats in the April election but needs allies to form a cabinet, is under strong pressure from the US and other countries to swiftly build a more inclusive government to undermine support for the insurgency.

Maliki confirmed this week that he would support the constitutional deadlines to set up a new government, after pressure from US Secretary of State John Kerry, who flew to Baghdad for emergency crisis talks to urge him to act.

The 64-year-old Shi'ite Islamist is fighting for his political life in the face of an assault that threatens to dismember his country. Sunni, Kurdish and rival Shi'ite groups have demanded he leave office, and some ruling party members have suggested he could be replaced with a less polarising figure, although close allies say he has no plan to step aside.

Fighters from ISIL have been assisted in their advance by other, more moderate Sunni armed groups who share their view that Sunnis have been persecuted under Maliki.

Washington hopes that armed Sunni tribal groups, which turned against al Qaeda during the US "surge" offensive of 2006-2007, can again be persuaded to switch sides and back the government, provided that a new cabinet is more inclusive.

British Foreign Secretary William Hague arrived in Baghdad on Thursday, reinforcing the international push for Maliki to speed up the political process.-Reuters