Iraqi security forces on Wednesday retook control of more areas from the Islamic State (IS) militants in and near the oil refinery town of Baiji in Salahudin province, as part of a major offensive to free the northern parts of the province, a provincial security source said.

The troops and allied Shiite and Sunni paramilitary units, known as Hashd Shaabi, carried out attacks at dawn after heavy overnight bombardment on the IS positions in town of Baiji, some 200 km north of the Iraqi capital of Baghdad, nearby Baiji's oil refinery, and the small town of Seiniyah, just west of the battleground in Baiji, the source told Xinhua on condition of anonymity.

After hours of fierce clashes with IS militants, the troops made slow progress in several districts in central Baiji, while the troops near Seiniyah recaptured a small airbase and cut a major supply route between Salahudin province and the adjacent Anbar province, the source said.

The troops, who were covered by U.S.-led coalition and Iraqi aircraft, also regained control of an army headquarters in north of the oil refinery of Baiji, which has been used by IS militants as a headquarters, the source said, adding that fierce clashes still underway inside the refinery between the troops and IS militants who seize large parts of it.

The battles in Salahudin province are part of second phase of major offensive announced late on Monday by Iraqi Prime Minister Haider al-Abadi, who is also Commander-in-chief of Iraqi armed forces, aimed at freeing towns seized by IS group since June 2014 in the northern part of Salahudin province.

In April, Iraqi security forces freed the provincial capital city of Tikrit, after weeks of heavy fighting and air strikes by the U.S.-led international bombing campaign against IS, as well as efforts by Shiite militias of Hashd Shaabi groups.

During the past few months, the security forces and allied militias, backed by Iraqi and U.S.-led coalition aircraft, were fighting with extremist militants in Baiji and the nearby Iraq's largest oil refinery, but the two sides have been involved in fierce tug-of-war battles.

Salahudin, a predominantly Sunni province with its capital of Tikrit, some 170 km north of the Iraqi capital of Baghdad, is the hometown of former President Saddam Hussein.