Iraqi troops aided by Shiite and Sunni forces reached the center of Tikrit Tuesday in an offensive against the Islamic State for control of the city, the country’s prime minister announced.

“Our security forces have reached the center of Tikrit and they have liberated the southern and western sides and they are moving toward the control of the whole city,” Prime Minister Haidar al-Abadi said in a statement released by his office, according to Reuters.

Abadi said U.S. and Iraqi-led airstrikes have helped reclaim ground.

But Abdul-Wahab al-Saadi, the commander of the operation, said his forces fighting in from the west were still 325 yards from the center of Tikrit.

A defense official told Fox News that hundreds of ISIS fighters are holed up in the center of Tikrit and have booby-trapped their positions, making it difficult to retake the center of the city.

“[ISIS] has had a long time to build fortifications, plant bombs and make it very difficult on the advancing forces,” the official added.

An Associated Press reporter embedded with Iraqi security forces saw soldiers surround the iconic presidential palace in Tikrit on Tuesday, as well as surround the provincial government headquarters there.

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

The large-scale operation to recapture Saddam Hussein's hometown began on March 2 and is seen as a key step toward eventually recapturing Mosul, the country's second largest city, which lies further north.

Iraqi military commanders said Monday they retook a hospital in southern Tikrit. The officials, who spoke to The Associated Press on condition of anonymity because they were not authorized to brief journalists, said the military's three-pronged offensive is progressing toward the center of Tikrit slowly because of booby traps and suicide bomb attacks.

At least four suicide bombers tried to ambush one of their positions in southern Tikrit on Monday, but the attack was repelled.

The ground offensive to retake Tikrit has been waged by Iraqi troops and Shiite militias advised by Gen. Qassem Soleimani, commander of the Iranian Revolutionary Guard's elite Quds Force.

The leader of Iraq's Popular Mobilization Units, a collaborative force made up mostly of Shiite militias, also said Tuesday 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 Associated Press that his fighters participated in the southern offensive Tuesday and would be joining the offensive on the northern and western fronts shortly.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.