Iraq's Prime Minister Haider al-Abadi declared the "liberation" of Tikrit from the Islamic State group on Tuesday after a weeks-long operation that was Baghdad's biggest operation yet against the jihadists.

Abadi "announces the liberation of Tikrit and congratulates Iraqi security forces and popular volunteers on this historic milestone", his official Twitter account said, referring to the allied paramilitary groups that have played a major role in the fighting.

"Iraqi forces reached the centre of Tikrit, raised the Iraqi flag and are now clearing the city," Abadi's spokesman, Rafid Jaboori, confirmed to AFP.

But it was not immediately clear on Tuesday if the Islamic State (IS) group had ended all resistance, and the massive challenge of defusing the explosives planted in roads and houses remained ahead.

Iraqi soldiers and police and the Popular Mobilisation units – paramilitary forces dominated by Shiite militias – began the battle to retake Tikrit on March 2.

They were able to recapture towns near the city and then surround it, but faced difficulties pushing into Tikrit and the operation stalled.

Over the objections of some militia forces, a US-led coalition began carrying out air strikes in support of the operation, prompting some of the groups to announce they were halting offensive operations.

Iran had been the main foreign backer of the operation for several weeks, and some Shiite militiamen accused Washington of attempting to hijack their victory.

In the end, the US carried out air strikes in spite of the objections and the militias took part in the final fighting despite Washington's role.

The Islamic State group spearheaded a major militant offensive that overran much of Iraq's Sunni Arab heartland last June, and large chunks of Nineveh and Anbar provinces are still under their control.

(FRANCE 24 with AFP)