A volley of rockets slammed into an Iraqi airbase north of Baghdad where US forces have been based, wounding four local troops, the Iraqi military said on Sunday.

Its statement said eight Katyusha-type rockets landed on Al-Balad airbase, wounding two Iraqi officers and two airmen.

Al-Balad is the main airbase for Iraq's F-16s, which it bought from the US to upgrade its air capacities.

The base had held a small US Air Force contingent as well as American contractors, but a majority had been evacuated following tensions between the US and Iran over the past two weeks, military sources told AFP.

"About 90 per cent of the US advisers, and employees of Sallyport and Lockheed Martin who are specialised in aircraft maintenance, have withdrawn to Taji and Erbil after threats," one of the sources said.

"There are no more than 15 US soldiers and a single plane at al-Balad," the source added.

Military bases hosting US troops have been subject to volleys of rocket and mortar attacks in recent months that have mostly wounded Iraqi forces, but also killed one American contractor last month.

That death set off a series of dramatic developments, with the US carrying out strikes against a pro-Iran paramilitary group in Iraq as well as a convoy carrying top Iranian and Iraqi commanders outside Baghdad airport.

Pro-Iran factions in Iraq have vowed revenge for those raids, even as Iran said it had already responded in "proportion" by striking another western airbase where US soldiers are located.

Rocket attacks against Baghdad's high-security Green Zone, where the

US and other embassies are based alongside international troops, are still taking place.