TIKRIT, Iraq, May 26 (Xinhua) -- Four Iraqi police officers were killed on Saturday during an operation to hunt down remnants of the Islamic State (IS) militant group in the central province of Salahudin, a provincial police source said.

In the early morning, a joint force from Iraqi army, provincial police and a paramilitary Hashd Shaabi brigade, backed by helicopter gunships, advanced from two directions to clear the rugged Mteibijah area near the border between the provinces of Salahudin and Diyala, Mohammed al-Obeidi told Xinhua.

During the operation, a huge roadside bomb explosion hit a police vehicle, killing Mohammed al-Jubouri, police chief of the town of Alam, some 15 km east of the provincial capital Tikrit, his deputy Salih Samir and two policemen, Obeidi said.

The operation in Mteibijah was designed to take control of the entire area, where hundreds of IS militants are believed to hide, in order to prevent their attacks against civilian and military targets in Salahudin and Diyala, he added.

However, the vast rugged land and mountains in the predominately Sunni Arab province of Salahudin have made it difficult for Iraqi troops to dislodge the extremist militants from Mteibijah.

Hundreds of IS militants earlier fled their former bases in the key cities of Salahudin, including Tikrit, after the Iraqi forces cleared these areas during major anti-IS offensives.

Late in 2017, Iraqi Prime Minister Haider al-Abadi, also the commander-in-chief of Iraqi forces, officially declared full liberation of Iraq from IS militants after Iraqi forces recaptured all the areas once seized by the extremist group.

However, some IS remnants have since melted in urban areas or fled to deserts and rugged areas in the country, carrying out attacks from time to time against security forces and civilians.