At least five attacks have targeted the Balad airbase, where other US companies including Sallyport are also present, since the start of the year.

At least three foreign subcontractors and one Iraqi subcontractor have been wounded.

The attacks are rarely claimed, and when they are it is by obscure groups that experts say are a facade for Iran-backed Iraqi factions.

"On Monday morning, 72 Lockheed Martin technicians left," a high-ranking Iraqi military official told AFP, while a second confirmed the move.

"The technical team in charge of maintenance of the F-16s left the Balad base for Arbil," the capital of Iraq's Kurdistan region, the first source added, requesting anonymity.

Baghdad had sent its national security adviser Qassim al-Araji to the Balad base last week to try to reassure the American firm, days after the latest salvo.

Tahsin al-Khafaji, spokesman for Iraq's Joint Operations Command, said Lockheed Martin would "continue to advise the Iraqi air force, even remotely", citing contractual obligations.

The United States has provided Iraq with 34 F-16s, all stationed at Balad. It has also trained Iraqi pilots, while American contractors have been in charge of the fleet's upkeep.

Arbil was long considered safer than the rest of Iraq, but the situation has changed recently and Washington has deployed a C-RAM rocket defence system as well as Patriot missiles there, as it has done in Baghdad to protect its troops and diplomats.

In mid-April, pro-Iran fighters sent an explosives-packed drone crashing into Arbil airport in the first reported use of such a weapon against a base housing US troops in Iraq.

The Pentagon has warned that attacks against the US-led coalition rose in the first three months of this year.

"In Iraq, Iran-aligned militias increased their attacks targeting coalition positions and assets this quarter, prompting a temporary departure of US contractors supporting Iraq's F-16 program," it said in a report to Congress released earlier this month.

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LOCKHEED MARTIN