Iran has entered the fight to retake a major Iraqi oil refinery from Islamic State militants, contributing small numbers of troops – some operating artillery and other heavy weapons – in support of advancing Iraqi ground forces, US defense officials said.

The US and its allies, meanwhile, have staged 22 air strikes on Isis targets since Friday, including four near Ramadi, the city taken by the militants this week, the US military said on Saturday. Coalition forces also attacked five Isis sites in Syria between Friday and Saturday.

Two US officials said Iranian forces had taken a significant offensive role in the Beiji operation in recent days, in conjunction with Iraqi Shia militia. The officials were not authorized to discuss the matter publicly and spoke on Friday on condition of anonymity. […]