TIKRIT, Iraq - Iraqi security forces launched Tuesday a major offensive against militants of the Islamic State (IS) to free the city of Tikrit, the capital of Salahudin province, a provincial security source said.

Iraqi army and police forces backed by thousands of allied Shiite and Sunni militias entered outer neighborhoods of the city, some 170 km north of the Iraqi capital of Baghdad, from the south, west and north, while heavy clashes with the extremist militants are underway, the source told Xinhua on condition of anonymity.

The troops were also backed by dozens of tanks and armored vehicles and covered by heavy artillery shelling and Iraqi aircraft, the source said.

For the past few days, the city has been under siege by the Iraqi forces and continuing heavy bombardment on IS positions inside the city, the source said.

The battles to free Tikrit started one day after the troops retook control of the last IS stronghold in town of al-Alam, east of Tikrit.

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

Large parts of Salahudin province have been under IS control since June 11, a day after bloody clashes broke out between Iraqi security forces and the IS group, which took control of the country's northern city of Mosul and later seized swathes of territories in Nineveh and other predominantly Sunni provinces.