BAGHDAD, July 19 (Xinhua) -- Iraqi security forces Thursday thwarted an attack by four suicide bombers from the Islamic State (IS) in Iraq's central province of Salahudin, a provincial security source told Xinhua.

The four would-be suicide bombers, wearing explosive belts, attacked early in the morning a security checkpoint in west of the oil-refinery town of Baiji, some 40 km north of Salahudin's capital Tikrit, Major Salah Salman from Salahudin's Operations Command told Xinhua.

The security members opened fire on the attackers before reaching the checkpoint and killed them all. One of the suicide bombers managed to blow up his explosive belt, wounding six security members.

In a separate incident, two Sunni tribal fighters were killed when IS militants attacked their checkpoint in the early hours of the day at a village near the city of Shirqat, some 110 km north of Tikrit, the source said.

The key cities of Salahudin province, including the provincial capital Tikrit, some 170 km north of the Iraqi capital Baghdad, were captured by IS militants in June 2014, but the Iraqi security forces have freed the province during major anti-IS offensives.

On Dec. 9, 2017, Iraqi Prime Minister Haider al-Abadi officially declared full liberation of Iraq from IS militants after Iraqi forces recaptured all the areas once seized by the extremist group.

However, small IS groups and individuals regrouped in the rugged areas, carrying out attacks against the security forces and civilians despite operations from time to time.