Defense Secretary Ash Carter said he is willing to send American helicopters and troops to Anbar province to help the Iraqi military forces close in on and seize Ramadi from Islamic State militants.

"The United States is prepared to assist the Iraqi army with additional unique capabilities to help them finish the job, including attack helicopters and accompanying advisers if circumstances dictate and if requested by Prime Minister [Hader al-] Abadi," Carter told lawmakers on Capitol Hill on Wednesday.

His comments came amid reports that the Iraqi security forces have advanced into downtown Ramadi and seized a key military operations center.

Hundreds of U.S. troops are deployed in Anbar province, but their mission has been limited primarily to inside-the-wire training activities at Al Asad Air Base and Taqaddum Air Base.

Carter's suggestion that U.S. forces and helicopters might support Iraqis closer to the front lines is the latest sign that the Pentagon and White House are intensifying military efforts to defeat the Islamic State group.

Last week, Carter revealed plans to send an "expeditionary targeting force" to Iraq that will include up to 100 U.S. special operations troops to focus on raids on Islamic State group targets in Iraq and Syria.

About 3,600 U.S. troops are authorized to be deployed in Iraq. The vast majority are focused on train, advise and assist missions, but during the past several months, some American advisers have been moved outside the wire and closer to Iraqi-led combat operations.

For operations in which U.S. troops accompany Iraqi forces onto the battlefield, as they did in Sinjar in November, the Americans typically remain at a rear position, advising Iraqi military commanders, standing by to provide medical evacuation and gathering intelligence that can develop targets for air strikes.

The U.S. has spent months training Iraqi forces in Anbar, conducting airstrikes almost daily and providing munitions like AT4s, the 84mm unguided shoulder-fired missiles that can stop truck bombs, Carter said.

Islamic State militants seized Ramadi in May. Carter said the Iraqis' progress this week comes after a "frustratingly long time."

Islamic State fighters in Ramadi are estimated to number 600 to 1,000, defense officials said.

The militants who have controlled the city for more than six months have set up a daunting defensive perimeter of mines, shooting positions and improvised explosive devices.