The mortar attacks -- which began in recent weeks -- underline the risks of an expanding role for the US military in Iraq, as hundreds of American troops deploy to train and advise the Iraqi army in its fight with the IS group.
About 320 troops -- mostly US Marines -- started working with Iraqi army units in recent days at the sprawling al-Asad base, which officials insisted was facing no imminent threat from the IS militants.
"It's fair to say al-Asad is coming under... regular harassment fires," spokesman Colonel Steven Warren told reporters.
The mortar and other indirect fire has been "wholly ineffective" so far, Warren said. "No US personnel, no US equipment have been impacted in any way."
It was the first time the Pentagon reported the mortar fire, which began "over the past few weeks," a US defense official told AFP.
US warplanes have carried out a steady stream of air raids near the Asad base against the IS militants, who have gained a firm foothold in the west. Anbar province is home to a mostly Sunni population that has become alienated from the Shiite-led government in Baghdad.
American commanders have described the military training as focused on "bare minimum basics" for the Iraqi soldiers. The instruction covers several topics, including coordinating air strikes from Iraqi or coalition planes, Warren said.
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INCREASING US FOOTPRINT
President Barack Obama plans to send up to 3,100 US troops to Iraq to help build up the Baghdad government's army in its fight with the IS group. But Obama has ruled out a direct combat mission for the American forces.
About 2,140 US troops are currently on the ground in Iraq, including the team at the Asad base -- one of five sites chosen for training efforts.
About 170 troops from the US Army's 1st Infantry Division have deployed to Taji, a vast base north of Baghdad. The training effort at Taji, which involves a six-week program for four Iraqi army battalions, began on December 27, according to Warren.
In the coming weeks, two other training sites will be launched by the US-led coalition, one in Arbil in the north and another in Besmaya, south of Baghdad.
The American troop contingent in Iraq also includes 800 forces guarding American diplomats and facilities.