Iraqi forces backed by Shiite militias and US-led airstrikes have driven "Islamic State" (IS) fighters from Tikrit's downtown, Iraqi Prime Minister Haider al-Abadi said in a statement on Tuesday.

Security forces had taken control of western and southern neighborhoods before closing in on the city center, the statement said.

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

Soldiers spent much of Tuesday locked in street battles with IS and there were differing accounts about how much of Tikrit was under Iraqi control. Army sources reported that soldiers had managed to take back the provincial council headquarters, the security headquarters, and the iconic presidential palace.

Army Lt. Gen. Talib Shaghati said at least 75 percent of the city had been recaptured, while Ammar Hikmat, deputy governor of Salahuddin province, said more than 40 percent was under Iraqi control.

"I think the whole city will be retaken within the coming 24 hours," Hikmat said.

The militants reportedly planted bombs in the embattled city - hometown of former dictator Saddam Hussein - and were using snipers in an effort to slow the advance of Iraqi forces.

Road to Mosul

The push to take back Tikrit was launched on March 2 with some 20,000 Iraqi troops and pro-government Shiite paramilitaries. From March 25, the army began receiving support in the form of US-led airstrikes. Some of the militia groups who had suspended their involvement in protest of the US bombing returned to the battle on Tuesday.

IS insurgents seized Tikrit, 160 kilometers (100 miles) north of Baghdad, last June during a rapid advance across northern Iraq. Regaining control of the city would give Abadi's government a strategic hub for launching an offensive to recapture Iraq's second biggest city, Mosul, which was also overrun by IS last year.

US military officials say a coordinated military mission to retake Mosul, involving up to 25,000 Iraqi troops, could begin within two months.

nm/jr (Reuters, AP, AFP, dpa)