A US air strike killed two "high-value" Islamic State fighters in Iraq, while separately Iraqi security forces have started clearing jihadists from the town of Rutba, a US military spokesman said Wednesday.

Baghdad-based military spokesman Colonel Steve Warren said the May 13 strike on a building killed Abu Hamza, Abu Safiya and a third IS fighter.

"Hamza was a former (Al-Qaeda in Iraq) member who we know planned and conducted attacks against Americans during Operation Iraqi Freedom," Warren said in a video call to Pentagon reporters.

Safiya had been "responsible for staging chemical attacks in the Euphrates River Valley," he added, though did not provide details.

The strike was part of an ongoing US-led, anti-IS air campaign across Iraq and Syria that began in August 2014.

It occurred far from the town of Rutba, which is located in the west of Iraq in Anbar province and has been held by the jihadist group since 2014.

Warren said about 1,000 Iraqi troops had spent weeks preparing to retake the town.

"Although a small town, Rutba has outsized strategic value," Warren said.

"Rutba lies on the main route between Baghdad and Jordan, and opening it will impact the economies of both Iraq and Jordan, and will deny ISIL a critical support zone as well," he added, using another IS acronym.

About 200 IS fighters had been holding Rutba, but they put up scant resistance.

"A lot of the enemy, frankly, ran away when they saw this force coming," Warren said.

The IS group has suffered a string of setbacks and lost between 30 and 35 percent of the overall territory they once held across Iraq and Syria.

US-led air strikes have killed more than 120 "high-value" figures, and attacks on IS cash hoards and the group's illicit oil-smuggling trade have deprived the jihadists of hundreds of millions of dollars in funding.

But despite the squeeze, IS jihadists have claimed responsibility for a string of devastating attacks in and around Baghdad that have killed more than 140 people in the city over the past seven days.

"They appear to have chosen to revert to some of their terrorist roots," Warren said.

"This is an enemy who has not found success in some time, so what they are trying to do is find a way to throw a punch that actually can land," he added.

Despite the attacks, the US military did not assess more troops were currently needed to protect US facilities such as the embassy.