Sunni insurgents battling forces loyal to Iraqi Prime Minister Nuri al-Maliki broke into a military base in Diyala province in northeast of Baghdad on Thursday, a security source and a local official said.
They said militants fought their way into the base on the edge of Muqdadiya town, 80 km (50 miles) northeast of the capital. The northern side of the town was already under their control before Thursday's clashes.
The area around Muqdadiya has seen sustained fighting in recent weeks since fighters led by the hard-line Islamic State, an offshoot of al Qaeda, seized the northern city of Mosul and swept south towards Baghdad.
The security source said that the insurgents on Thursday included hundreds of non-Iraqi Arab fighters, heavily armed and some riding in tanks. Negotiations were underway involving local tribal sheikhs for a truce.
A local official confirmed the militants had breached the site, although he said they would be repulsed. "They were able to control part of the base but we will retake it from them," he said, requesting that his identity shall not be reveled.
A military spokesman was not immediately available to comment.
Thousands of volunteers have signed up to join militia forces fighting alongside the army and security forces, many of whom retreated or threw off their uniforms in the face of the
initial militant offensive.
Maliki has vowed to rout the insurgents.