TIKRIT, Iraq, March 12 (Xinhua) -- Iraqi security forces on Thursday continued their offensive to free Tikrit, the capital city of Salahudin province, from the Islamic State (IS) militants, as troops cautiously rushed into streets buildings planted with bombs, security source and official said.

After two days of fierce clashes, Iraqi troops and government-backed Shiite and Sunni militias gained some ground by seizing parts of the city, which is located some 170 km north of the Iraqi capital Baghdad, the source told Xinhua on condition of anonymity.

The troops, supported by dozens of tanks, armored vehicles, heavy artillery shelling and aircraft, are moving slowly and cautiously amid heavy clashes with the extremist militants, including a large number of IS snipers, while they also have to deal with hundreds of roadside bombs and booby-trapped houses, the source said.

Iraqi Defense Minister Khaled al-Obeidi told reporters near Tikrit that the Iraqi forces and the allied militias, known as Hashid al-Shaabi, or Popular Mobilization, are moving to the second phase of the plan to liberate Tikrit on Thursday, "which will be decisive to free the city from the IS militants."

The minister, however, did not specify the time for the next phase of battle, as leaders of the security forces want to avoid casualties.

"We are very keen not to cause heavy casualties among our troops. Time is in our side, we don't need to rush. What is important is to achieve the goal with minimum of losses," the minister said.

The first phase of Tikrit battle is to destroy the main defensive fortifications of the IS militants and to win footholds in the city which will be used as a launching pad for further actions, according to the minister.

Meanwhile, in early Thursday morning, the IS militants blew up a strategic bridge on Tigris River that links Salahudin province with the neighboring province of Kirkuk, a security source told Xinhua on condition of anonymity.

The 500-meter bridge was bombed when the Iraqi forces were advancing closer from the east side of the river, trying to cut the main road on the west side that links Salahudin province to Mosul, which is vital for the IS group, in the north.

Earlier on Tuesday, the Iraqi forces launched their major offensive to enter the outer neighborhoods of Tikrit, but were slowed by fierce clashes with IS militants and roadside bombs.

Some 30,000 Iraqi troops and thousands of allied Shiite and Sunni militias have been involved in a week-long operation to recapture Tikrit and other key towns and villages in the northern part of Salahudin province from IS militants.

Large parts of the province have been under IS control since June 2014, after bloody clashes broke out between Iraqi security forces and the extremist group.

The IS has taken control of the country's northern city of Mosul and later seized swathes of territories in Nineveh and other predominantly Sunni provinces.