Baghdad: Daesh militants attacked government forces and their Shiite militia allies on Saturday, killing 11 near the city of Baiji as part of the battle for control of Iraq’s biggest refinery, army and police sources said.

Four suicide bombers in vehicles packed with explosives hit security forces and the local headquarters of the Shiite militias in the area of Al Hijjaj, 10 km to the south of Baiji town, near the refinery, sources at the nearby Tikrit security operations command said.

Iraqi government forces and Iranian-backed Shiite militias face Daesh on several fronts in Iraq, a major oil producer and Opec member. They include areas around Baiji refinery, north of Baghdad, and the city of Ramadi west of the capital, seized last month by the militants.

Ramadi is the provincial capital of Anbar Province, Iraq’s Sunni heartland.

On Wednesday, US President Barack Obama ordered the deployment of 450 more US troops to Anbar to advise and assist Iraqi forces being built up to try to retake territory lost to Daesh.

Iraq has been struggling to find a formula for stability since the last US troops withdrew in 2011.

Daesh’s drive, hardline views and ambitions to create a self-sustained caliphate where opponents are executed or beheaded, have exacerbated a sectarian conflict.

The Iraqi army depends heavily on support from the umbrella Shiite militia group Popular Mobilisation Front in the face of advances from Daesh.

Unlike its predecessor in Iraq, Al Qaida, Daesh has shown a capability to hold territory it captures. It now controls about a third of Iraq in the north and the west, as well as large parts of neighbouring Syria.

Daesh also holds territory in Libya and has militant sympathisers in Egypt, the most populous Arab state.