US military informed Iraqi officials that they would begin evacuating about half of the 800 employees.

U.S. forces are preparing to evacuate hundreds of staff working for Lockheed Martin Corp and Sallyport Global from an Iraqi military base where they work as contractors, three Iraqi military sources said on Friday.

Nearly 400 contractors from the two firms were getting ready to leave Balad military base, which hosts U.S. forces some 80 km (50 miles) north of Baghdad, over "potential security threats." Their departure was imminent, the sources said.

The sources did not give any details about the security threats.

The sprawling Balad base was hit by three mortar shells last week. No one claimed responsibility for the attack.

The U.S. military informed Iraqi officials that they would begin evacuating about half of the 800 employees who work for both companies at Balad, said a military official with knowledge of the base's daily operations.

The official said the evacuation would take about 10 days.

Two other military sources said the evacuation would take place in two stages and would be carried out by military aircraft.

"Americans informed us that they will only keep limited, necessary staff who work closely on the maintenance of Iraqi F-16 war planes." Lockheed Martin began delivering the first F-16s to Iraq in 2014.

The sources said the evacuation could start at any moment.

Two other Iraqi bases hosting U.S. forces have been hit by rockets in the past week in unclaimed attacks. On Wednesday, a rocket attack hit near a site used by U.S. energy company Exxon Mobil near the southern city of Basra.

Local officials blamed Iran-backed Shi'ite militias for the Basra incident. Iran has not commented on the Iraq incidents but has strongly rejected accusations by Washington that it was behind several attacks on oil tankers in the Gulf in recent weeks.

The uptick in violence comes amid rising regional tension between the United States and Iran.

Washington has ramped up sanctions pressure on Tehran since last year and several violent incidents in the Gulf have been blamed on the rising tension.

President Donald Trump on Friday said he had called off a U.S. strike on Iran at the last minute.

In Iraq, Iran backs several Shi'ite Muslim militias which have positions close to U.S. military installations.

Those militias have not publicly commented on the recent incidents.

Sunni extremist group Islamic State is also trying to stage a comeback in Iraq and has mostly used hit-and-run insurgency tactics against Iraqi forces in recent months.