An Iranian deputy foreign minister denied allegations that Iranian forces were fighting in Iraq to regain control over the Beiji oil refinery.

U.S. military strikes have supported Iraqi troop activity against the group calling itself the Islamic State in many of the country's western Sunni-dominated provinces. The militant group claimed a brief hold over the Beiji oil refinery in Saladin province, north of Baghdad, last year and heavy fighting has been reported recently in and around the area.

The Islamic State has claimed control over Ramadi, the capital of neighboring Anbar province.

U.S. defense officials said last week there were a small number of Iranian forces working on the ground in coordination with Shiite militias to retake the Beiji refinery. Hossein Amir Abdollahian, a deputy minister for Arab and African affairs, denied the claims.

"We have sent no military forces to Iraq," he said Monday from Moscow.

Washington last year said disrupting the ability of the Islamic State to raise funds through oil revenue was a key U.S. military objective. In Ramadi, U.S. Defense Secretary Ash Carter told CNN during the weekend Iraqi forces "showed no will to fight" the Islamic State.

Analysis from consultant group IHS last year found the Islamic State insurgency wasn't having much of an impact on the overall oil sector in Iraq.

The Organization of Petroleum Exporting Countries said in its market report for May its total crude oil production of 30.8 million barrels per day was 18,000 bpd above the previous month.

"Crude oil output increased mostly from Iraq and Iran," the 12-member group said.