A banner is placed at a IS checkpoint, in Anbar province, July 22, 2014. The banner reads, ”Islamic State, State of Anbar, City of Rawah.”

BAGHDAD — “Islamic State” insurgents took control on Thursday of most of the western Iraqi town of al-Baghdadi, threatening an air base where U.S. Marines are training Iraqi troops, officials said.

Al-Baghdadi, about 50 miles northwest of Ramadi in Anbar province, had been besieged for months by the extremist Islamist militants who captured vast swathes of northern and western Iraq last year.

“Ninety percent of al-Baghdadi district has fallen under the control of the insurgents,” district manager Naji Arak told Reuters by phone.

Militants attacked al-Baghdadi from two directions earlier in the day and then advanced on the town, intelligence sources and officials in the Jazeera and Badiya operations commands said.

The officials said another group of insurgents then attacked the heavily-guarded Ain al-Asad air base, three miles southwest of the town, but were unable to break into it.

About 320 U.S. 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.

Pentagon spokeswoman Navy Commander Elissa Smith confirmed the fighting in al-Baghdadi. She said there had been no direct attack on the air base, adding: “There were reports of ineffective indirect fire in the vicinity of the base.”

An Iraqi defense ministry spokesman declined to comment on the situation in Anbar.

Most of the surrounding towns in Anbar fell under IS control after the group’s rapid advance across the Syrian border last summer.

Elsewhere in Iraq, five civilians were killed when bombs went off in two towns south of Baghdad, police and medical sources said. Such attacks are not uncommon in and around the capital.