As many as 25 ISIS fighters, some in Iraqi Army uniforms, infiltrated an air base where U.S. Marines are training Iraqi troops, the Pentagon said Friday.

Rear Admiral John Kirby, the Pentagon press secretary, told reporters that most of the attackers, some of whom were suicide bombers, were killed by Iraqi security forces guarding the al-Asad air base.

He said no U.S. troops were involved and Iraqi forces suffered no casualties.

According to ABC news, the ISIS fighters attacked Iraqi forces inside the base, where senior Iraqi military leaders were present.

The base, in remote western Iraq has been under attack by ISIS since the militant group took over the nearby city of Al-Baghdadi, about 50 miles northwest of Ramadi in Anbar province.

Militants from the jihadist group had attacked the base and the nearby town of al-Baghdadi a day earlier, leading to sporadic clashes in the town overnight.

Al-Baghdadi has been besieged for months by Islamic State, which captured swathes of northern and western Iraq last year, prompting a campaign of U.S.-led air strikes and the deployment of hundreds of U.S. military advisers to the country.

A U.S. defense official said the Iraqi forces had stopped the attack and re-secured the facility

"Coalition forces were several kilometers from the attack and at no stage were they under direct threat from this action," the official said.

The Marines are training members of the Iraqi 7th Division at the base, which has been struck by mortar fire on at least one previous occasion since December.

Prime Minister Haider al-Abadi, a moderate Shi'ite Islamist who since taking office in September has promised to support the neglected Sunni minority community, said the army was prioritizing the western Sunni province in its fight against Islamic State.

"The Iraqi government and security forces are extremely concerned with Anbar, its defense and the protection of its residents from Daesh," he said on his official Facebook page, using a pejorative acronym for the ultra-radical Sunni group.

An Iraqi military official in Baghdad told Reuters the insurgents had taken advantage of a lull in the air strikes caused by poor weather to launch the offensive, but that weather had since improved.

The U.S. military said the United States and its coalition partners conducted seven air strikes against Islamic State militants in Iraq between early Thursday and early Friday, including five strikes about 15 km east of Ain al-Asad base.

The Iraqi official said Islamic State had been cleared from most of al-Baghdadi, with the remaining fighting centered around a police station.

That conflicted with reports from a tribal leader who said the jihadists were still in control of much of the town.

Ongoing clashes and poor communications in the area made it difficult to confirm such reports.