A volley of rockets slammed into an Iraqi airbase north of Baghdad where U.S. 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 U.S. to upgrade its air capacities.

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

“About 90 percent of the U.S. advisers, and employees of Sallyport and Lockheed Martin who are specialized in aircraft maintenance, have withdrawn to Taji and Irbil after threats,” one of the sources said.

“There are no more than 15 U.S. soldiers and a single plane at al-Balad,” the source added.

Military bases hosting U.S. 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 U.S. 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 U.S. soldiers are located.

Rocket attacks against Baghdad’s high-security Green Zone, where the U.S. and other embassies are based alongside international troops, are still taking place.