Iran has reportedly sent troops to Iraq to fight ISIS militants, U.S. defense officials said, according to the Associated Press.

The Islamic Republic, which sent small troops to allow Iraqi forces retake the major oil refinery of Beiji, also sent heavy weapons for Iraqi ground forces, one of the officials said.

Iran has stepped up its efforts in the Beiji Operation in recent days, the two officials, who spoke on condition of anonymity, said.

U.S. President Barack Obama said his country does not oppose the intervention of Iran in the Iraqi conflict as long as it operates under the control of Iraq’s government.

On Friday, a U.S. military statement said that Iraqi security forces and federal police have made “steady, measured progress” in regaining some parts of Beiji refinery compound.

The statement added that all forces involved in Beiji are “aligned with the government of Iraq” and under the control of Iraqi security forces.

However, it did not mention Iran’s role on the Beiji operation.

ISIS launched their fiercest attack on the Beiji refinery last month and now control large parts of the complex, according to Reuters.

The latest fighting has inflicted damage that will take years to repair, rendering the refinery idle at a time when Iraq is short of cash due to plummeting oil prices.

The refinery was liberated once before in November, but security forces failed to secure the area and deployed to other fronts, leaving the refinery a soft target.