Iraqi troops are more than halfway through an operation to encircle ISIS militants in Ramadi, after which they will launch a final offensive to retake the Iraqi city, a U.S. military spokesman said on Friday. Air Force Colonel Pat Ryder, a U.S. Central Command spokesman, said Iraqi forces were making progress in the fourth week of their effort to isolate and cut off ISIS fighters, who captured Ramadi three months ago in their biggest victory this year. "The objective here is to cut off ISIS's lines of communications to prevent or limit their resupply and reinforcement," Ryder, using an acronym for the militant group, said in a telephone briefing with Pentagon reporters. He said Iraqi troops were engaged in "tough, dangerous work" to encircle the city and then prevent ISIS from bringing in more troops or supplies. The militants were trying to slow or stop Iraqi forces with hidden explosive devices, improvised explosives in vehicles, suicide bombers and fighters, Ryder said. The Iraqis have used armoured bulldozers and other specialised equipment to remove explosives. "The Iraqis are continuing to move forward, executing this complex operation as they had planned it," Ryder said. "They have ... a good plan. They're executing that plan at the timeline they have set for themselves." He said the aim was to completely encircle Ramadi before the operation's next phase: retaking the city, which is the capital of Anbar province. Ryder said the U.S.-led coalition had supported the Iraqi military with air strikes and by providing military officials to offer advice and assistance at joint operations centres. He said air strikes had targeted vehicles, weapons and equipment as well as enemy fighters. Central Command is based in Tampa, Florida.