TIKRIT, Iraq, March 1 -- Iraqi Prime Minister Haider al-Abadi announced on Sunday evening the commencement of major offensive to free the towns seized earlier by the Islamic State(IS) militants in Iraq's northern central province of Salahudin, a provincial official told Xinhua.

The order to launch the offensive came as Abadi, who is also the Commander-in-Chief of the Iraqi armed forces, paid a surprise visit to the provincial operations command in the city of Samarra, some120 km north of the Iraqi capital of Baghdad, said Sheikh Jasim al-Jbarra, the head of the security committee in Salahudin provincial council.

"Abadi has ordered the security forces to commence the offensive to liberate the northern parts of Salahudin province after his meeting with the provincial top security commanders and the provincial governor Ra'ed al-Jubouri," al-Jbarra told Xinhua by telephone.

Abadi also ordered the troops and allied Sunni and Shiite militias to avoid casualties among civilians and to preserve their properties, al-Jbarra said.

The offensive came after days of heavy shelling targeted the IS positions in the cities, towns and villages that were seized earlier by the extremist militants.

The military operations were designed to sweep all redoubts of the IS militants across the province in order to prevent the militants from maneuvering with their groups.

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

Security has been worsening in Iraq since June 10, when bloody clashes broke out between Iraqi security forces and Sunni militants, who took control of the country's northern city of Mosul and the later swathes of territories in Nineveh and other predominantly Sunni provinces.