Washington, May 8 (IANS): A refinery site in Baiji city of northern Iraq was captured by Islamic State (IS) militants recently and Iraqi security forces now control only 20 percent of the facility, a senior US official said.

The Baiji refinery site, which was not operated for months, has been heavily contested. But IS fighters have successfully moved into several refinery areas in recent days, and are keeping the Iraqi forces from being re-supplied; CNN quoted the US official as saying.

IS now controls 'the majority" of the refinery the official said. While Iraqi forces have lost significant ground there in the last few days, the official described the battlefield situation as "highly contested" and said Iraqi forces have suffered steady losses leading to the current situation.

Iraqi forces include Army, national police and counter-terrorism units at the site. If the Iraqis lose the refinery "they will have to find a way to re-take it," the official said.

Coalition fighter and US jets have been able to strike IS fighting positions and supply routes around the refinery, but are not striking inside, the official said.

The reason, in part, was to preserve as much of the infrastructure as possible for the future. But the official emphasised in key areas IS controls, US airstrikes can only have a limited impact against IS which can move fighters into areas in small, not easy to find groups.

The refinery was considered strategic because it is key infrastructure and it is located on a major approach route to the city of Mosul, which Iraq forces hope to take back from IS in the coming months. However, the refinery has been non-operational for some time.