BAGHDAD (Reuters) - 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, with the battle against Islamic State and widespread sectarian bloodletting severely hampering efforts to rebuild the economy.
Islamic State's drive, its hardline views and ambitions to create a 'caliphate' where opponents are executed or beheaded, have exacerbated Iraq's sectarian conflict.
In eastern Iraq, tensions between Kurdish and Shi'ite forces ran high on Saturday for a second consecutive day. The two sides have in the past joined forces against Islamic State but competition for territory can sometimes undermine cooperation.
Trouble erupted when Kurdish peshmerga fighters began digging a trench to separate two towns in Diyala province.
On Saturday, clashes flared anew, police sources said, adding that four Shi'ite militiamen and two Kurdish peshmerga fighters had been wounded.
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, Islamic State holds territory it captures, while also conducting suicide bombings and beheadings. It now controls a third of Iraq, as well as large parts of neighboring Syria.
Islamic State holds territory in Libya and has militant sympathizers in Egypt, the most populous Arab state.
Reporting by Baghdad bureau; Writing by Michael Georgy; Editing by Gareth Jones