Iraqi security forces and paramilitaries deploy, on May 26, 2015, in al-Nibaie area, north-west of Baghdad, during an operation aimed at cutting off ISIS in Anbar.

The fight to retake Ramadi from ISIS militants has been on an “operational pause” and Iraqi troops weren’t trained to deal with the group’s battlefield techniques, a U.S. official said Thursday.

U.S. military spokesman Colonel Steve Warren, speaking via video from Baghdad, told reporters that the effort had been delayed in part because of record summer temperatures but also because of the way ISIS fighters had defended the city, which they seized in mid-May.

Iraq had planned to quickly retake Ramadi, but Warren acknowledged the fight had been tough.

“Ramadi has been a difficult fight,” he said. “I feel like we are coming out of what was essentially was an operational pause.”

He said ISIS had built “defensive bands” around the city – essentially fields littered with Improvised Explosive Devices (IEDs).

“They are using these IEDs almost as landmines to create these minefields, which they can then cover with (gun)fire,” Warren said.

“This is not what we trained the Iraqi army back in the early and middle 2000s to fight against. We trained and built a counterinsurgency army, and this is much more of a conventional fight.”

He said U.S. experts had put together some special training to deal with the challenge.

“This is a specific skill and it’s not a skill the Iraqis had had to exercise before and it’s not a skill we had taught them,” he said.

In order to get through these minefields, Warren said the Iraqi army was ordering bulldozers and “explosive line charges” that can be used to detonate a minefield.

Warren added that U.S. advisers were encouraging Iraqi generals to complete the task of retaking the city.

“We are all urging them to begin with the utmost haste to finish this fight in Ramadi,” he said. “It’s a very important fight and it needs to be finished.”

ISIS seized large areas of Iraqi territory in a June 2014 offensive, and a U.S.-led coalition is carrying out daily air strikes against the militants to assist Iraqi forces, which have made little progress on the ground in recent weeks.