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

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

And Abadi's spokesman Rafid Jaboori told AFP that: "Iraqi forces reached the center of Tikrit, raised the Iraqi flag and are now clearing the city."

But it was not immediately clear if resistance in the city by the Islamic State (ISIS) group had ended entirely, and the massive challenge of defusing explosives planted in roads and houses remains ahead.

Iraqi soldiers and police and 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 Tikrit and then surrounded it, but faced difficulties pushing into the city and the operation stalled.

Over the objections of militia forces, a U.S.-led anti-ISIS 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 militiamen accused Washington of attempting to hijack their victory.

In the end, the U.S. carried out airstrikes in spite of the presence of militias and the militias took part in the recent fighting despite Washington's role.

ISIS 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.