TIKRIT, Iraq (AP) — Iraqi forces battled Islamic State militants holed up in downtown Tikrit, going house to house Tuesday in search of snipers and booby traps, and the prime minister announced security forces had reached the city's center.

In a Twitter statement, Prime Minister Haider al-Abadi announced the "liberation of Tikrit" and congratulated Iraqi security forces on their "historic milestone." But an official statement from his office said the forces "hoisted the Iraqi flag" over the Salahuddin provincial headquarters in Tikrit and are moving to control the entire city.

Abdul-Wahab al-Saadi, the commander of the Salahuddin operation, said his forces fighting from the west were still 300 meters (325 yards) from the center of Tikrit.

Extremists from the Islamic State group seized Saddam Hussein's hometown last summer during its lightning advance across northern and western Iraq. The battle for Tikrit is seen as a key step toward eventually driving the militants out of Mosul, Iraq's second-largest city that is farther north.

Street-by-street fighting raged into the afternoon, and estimates differed widely on how much of this strategic city on the banks of the Tigris River that Iraqi forces held. Army Lt. Gen. Talib Shaghati said at least 75 percent of Tikrit had been recaptured. Ammar Hikmat, deputy governor of Salahuddin province, said more than 40 percent was under Iraqi control.

"Our security forces are now pushing forward toward the presidential complex and have already entered parts of it," Hikmat said. "I think the whole city will be retaken within the coming 24 hours."

An Associated Press reporter embedded with Iraqi security forces saw soldiers surround the iconic presidential palace, and they also surrounded the provincial government headquarters. Soldiers worked to detonate bombs remotely, while federal police went house to house looking to arrest militants or identify booby traps that may slow the offensive.

Interior Ministry spokesman Brig. Gen. Saad Maan Ibrahim said at least 40 militants were killed Tuesday, and Iraqi forces dismantled 300 roadside bombs. He said the federal police, backed by allied militias, were able to clear the government compound, the Tikrit provincial council headquarters, its security headquarters and the presidential palace.

The leader of Iraq's Popular Mobilization Units, a collaborative force made up mostly of Shiite militias, also said his fighters had rejoined the Tikrit operation, less than a week after announcing a boycott over U.S. involvement.

Abu Mahdi al-Mohandis told the AP that his fighters participated in the southern offensive and would be joining the offensive on the northern and western fronts shortly.

Iranian military advisers have been providing significant support since the offensive began March 2, arming and training the Iraqi Shiite militias, which have played a prominent role on the battlefield. Militiamen make up more than two-thirds of the force fighting the Islamic State group in Tikrit.

But the operation stalled until U.S. forces joined the offensive by launching airstrikes on March 25. Since then, Iraqi allied forces have moved in on the city, although they have been slowed by snipers and hidden bombs.

The Iraqi military has struggled to recover from its collapse against the Islamic State group in June, when commanders disappeared in the face of the extremists' advance. Pleas for more ammunition went unanswered, and in some cases, soldiers stripped off their uniforms and ran.

Recapturing Tikrit would be the biggest win so far for Baghdad's Shiite-led government. The city is about 130 kilometers (80 miles) north of Baghdad and lies on the road connecting the capital to Mosul. Retaking it will help Iraqi forces have a major supply link for any future operation against Mosul.

U.S. military officials have said a coordinated mission to retake Mosul likely will begin in April or May and involve up to 25,000 Iraqi troops. But the Americans have cautioned that if the Iraqis are not ready, the offensive could be delayed.

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Yacoub reported from Baghdad.