There were no casualties in the overnight attack, which follows one on the same base on Monday which wounded six members of Iraq's elite US-trained counterterrorism force, two of them critically, the army said.

Washington has expressed mounting concern about the flurry of attacks on US bases and diplomatic missions, several of which it has blamed on Shiite militia groups trained by its foe and rival for influence Tehran.

Security sources have linked at least one attack last week to Kataib Hezbollah, a powerful Shiite faction close to Tehran and blacklisted by Washington.

Iran holds vast sway in Iraq, especially among the more hardline elements of the Hashed al-Shaabi, a paramilitary force largely made up of Shiite militias.

A US defence official told AFP the rocket attacks made the Hashed a bigger security threat to American troops in Iraq than the Islamic State group, the jihadist movement which the US has vowed to help Baghdad wipe out.

On Friday, the United States imposed sanctions on three senior Hashed figures.

Tensions between Iran and the US have soared since Washington pulled out of a landmark nuclear agreement with Tehran last year and reimposed crippling sanctions.

Baghdad -- which is close to both countries and whose many security forces have been trained by either the US or Iran -- is worried about being caught in the middle.

US officials say they are considering plans to deploy between 5,000 and 7,000 additional troops to the region to counter Iran.