Islamic State jihadists are having to spend more effort defending key supply lines in Iraq due to US-led air strikes and pressure from local forces, the Pentagon said Friday.

The IS group's supply routes into Iraq from neighboring Syria have become a central focus of combat, with Iraqi government and Kurdish forces -- along with coalition warplanes -- seeking to disrupt and cut off the militants' access to weapons and equipment, spokesman Rear Admiral John Kirby told reporters.

"They're trying to protect what they can hold onto now, and... also we're seeing them put a lot more emphasis on protecting their lines of communications," Kirby said. "That's where they're putting their energy."

After nearly 1,700 air raids by US-led forces since August 8, the IS group's advance has been halted for the most part but the jihadists have held on to much of the territory they seized in Syria and Iraq last year.

US officials say the Iraqi government army is being trained and armed to stage a major counter-offensive later in 2015, but in the meantime, the international coalition is seeking to pile pressure on IS supply lines.

"If you look at the air strikes we're conducting, and you look at some of the operations that are being done by Iraqi and Kurdish forces, you can see that we're trying to disrupt their ability to do that, to preserve those lines of communication," he said.

"One of the keys for them to maintain the control they have is to be able to sustain themselves, and we're trying to make that as difficult as we can for them."

However, it was unclear if the IS group had faced serious difficulty resupplying its fighters as a result.

The comments came as the American military reported another round of air strikes Thursday against IS fighters, including six raids in Iraq. One air strike hit an IS bunker near Al-Qaim and two strikes were carried out near Sinjar in the northwest.

Kurdish peshmerga forces last month broke a months-long siege of Mount Sinjar, an advance that US officers say could eventually lead to disrupting a crucial route for the militants.