Iraqi Army soldiers patrol with new U.S.-made weapons and armored vehicles in an eastern suburb of Ramadi.

The United States is encouraging Iraqi forces to move to retake the city of Ramadi from Islamic State of Iraq and Syria (ISIS) militants “as quickly as possible” to deny them the chance to regroup, a U.S. military spokesman said on Friday.

The militants seized Ramadi, the capital of Anbar province, in May, extending its control over the Euphrates valley west of Baghdad and handing the Iraqi army its worst defeat since June 2014 when ISIS swept through northern Iraq.

The military setback has renewed questions about the ability of Iraq’s Shiite-led government in Baghdad to overcome the sectarian divide that has helped fuel the militants’ expansion in Iraq’s Sunni heartland.

Iraqi military and police, backed by Shiite militias, Sunni tribal fighters and U.S.-led coalition air strikes, are making slow progress in trying to retake the city, 100 kilometers west of Baghdad, from the Sunni militants.

“We acknowledge that the Iraqis have not made any significant forward movement recently,” said Col. Pat Ryder, a spokesman for U.S. Central Command, in a telephone briefing with Pentagon reporters.

“We continue to encourage ISF (Iraqi security forces) leaders to move as quickly as possible to prevent giving ISIL time and space to regroup and resupply,” Ryder said, using an acronym for Islamic State.

Ryder said the U.S. military believes the Iraqis have enough troops near Ramadi to move forward.

“There’s certainly significant defensive obstacles in their way,” Ryder said. “This is an Iraqi-led operation. They will move at their pace, and so we are supportive of their plan but it’s certainly something that we’re discussing with them.”

Shortly after they seized the city, ISIS fighters warned residents not to venture outside Ramadi because, they said, they had laid a web of bombs to hinder any incursion by government forces.