Islamic State militants attacked government forces and their Shi'ite 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 Shi'ite militias in the area of al-Hijjaj, 10 km (6 miles) to the south of Baiji town, near the refinery, sources at the nearby Tikrit security operations command said.

Iraqi government forces and powerful Iranian-backed Shi'ite militias face Islamic State 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 Islamic State, the ultra-hardline Sunni group that poses the biggest threat to Iraq since the fall of Saddam Hussein in 2003.

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

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

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

Islamic State'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 Shi'ite militia group Popular Mobilisation Front in the face of advances from Islamic State.

Unlike its predecessor in Iraq al Qaeda, the group holds territory it captures. It now controls about a third of Iraq in the north and the west, as well as large parts of neighboring Syria.

Islamic State also holds territory in Libya and has militant sympathizers in Egypt, the most populous Arab state. -Reuters