Islamic State militants in Iraq took partial control of a water dam and military barracks guarding it in the western Anbar province after fierce fighting through the night that continued yesterday, security sources and witnesses said.

The Iraqi government announced a new offensive this month to recapture parts of Anbar, Iraq’s Sunni Muslim heartland, from Islamic State, in an attempt to build on an earlier victory against the group in the central city of Tikrit.

But the hardline Sunni militants struck back by attacking Ramadi, the provincial capital of Anbar, prompting thousands of families to flee. They also hit Baiji, Iraq’s largest refinery last week.

Late on Friday, the insurgents attacked the security perimeter at al-Thirthar dam with explosive-laden vehicles and then battled army forces in clashes that continued on Saturday, police and army sources told Reuters.

The Iraqi government announced a new offensive this month to recapture parts of Anbar, Iraq’s Sunni Muslim heartland

Dozens of Iraqi troops were killed in the fighting, but poor communications in the area made it difficult to confirm a precise figure, said Athal al-Fahdawi, an Anbar provincial council member. Army sources said two senior officers were among the dead.

Officials from Iraq’s defence and interior ministries could not immediately be reached for comment. Security forces and Shi’ite paramilitaries have regained some ground in Iraq since Islamic State proclaimed a caliphate straddling the border of Syria and Iraq, but core Sunni territories such as Anbar remain under Islamic State control.

A video posted online yesterday by Islamic State’s official media organisation appeared to show its fighters moving about freely at al-Thirthar dam, which serves as a flood control for Baghdad and other cities.