The city of Ramadi will be a "tough nut to crack" for Iraqi Security Forces seeking to retake the capital of Anbar province and avenge a humiliating defeat by the Islamic State, the U.S. military in Baghdad said Wednesday.

The Iraqi Security Forces, or ISF, completed the encirclement of Ramadi last week by taking a bridge that was a key supply route for the Islamic State in Iraq and Syria, or ISIS, west of the city, and U.S. airstrikes Tuesday targeted ISIS tunnel complexes in an effort to clear the way for assaults, said Army Col. Steve Warren, a spokesman for Combined Joint Task Force-Operation Inherent Resolve in Baghdad.

The ISF "completed the isolation phase of the operation when they seized the Palestine bridge on Nov. 25. They are now poised to begin the clearing phase" but that operation will be difficult, although the Iraqi forces are estimated to outnumber the ISIS defenders by more than 10-1, Warren said.

He said about 600-1,000 ISIS fighters were thought to be holed up in the city that they took last May as ISF forces fled and abandoned their equipment.

The Iraqis have made "real progress" in recent weeks in shutting off Ramadi but "they haven't moved as fast as we'd like them to move," said Joint Chiefs Chairman Marine Gen. Joseph Dunford in testimony Tuesday to the House Armed Services Committee. He said that retaking Ramadi would likely be a process of months, and not weeks.

In a recent weekly briefing from Baghdad, Warren has described belts of improvised explosive devices, ISIS has thrown up around Ramadi to thwart the ISF, and the terrorist group's use of suicide vehicle borne improvised explosive devices, or VBIEDs, to break Iraqi lines.

"Ramadi's not an easy city to take," Warren said, noting that U.S. forces took about six months to occupy the city in 2007.

The ISIS defenses in Ramadi now "are more difficult than what we, the U.S. Army, faced years ago," Warren said. "It's a tough nut to crack."

An estimated 5,000-10,000 Iraqis remain in the city that was once home to more than 400,000 citizens. Residents of Ramadi reached by The Associated Press on Tuesday said that ISIS has set up checkpoints around the city to monitor their movements and prevent anyone from leaving.

The ISF dropped leaflets over the city Tuesday telling residents to leave but ISIS snipers reportedly targeted a designated evacuation route.

--Richard Sisk can be reached at Richard.Sisk@military.com.