Islamic State of Iraq and the Levant fires heavy machineguns during fighting in the northern Iraqi city of Samarra.

Islamic State of Iraq and the Levant fires heavy machineguns during fighting in the northern Iraqi city of Samarra. Photo: AFP

Washington: US companies were on Thursday evacuating hundreds of Americans working with the Iraqi government from a major airbase, US officials said, as Islamic militants swept towards Baghdad.

A US defence official confirmed that "a few hundred" American contractors from Balad airbase, 80 kilometres north of the capital, were being moved to Baghdad for security reasons.

"We can confirm that US citizens under contract to the government of Iraq in support of the US Foreign Military Sales (FMS) program in Iraq are being temporarily relocated by their companies due to security concerns in the area," State Department spokeswoman Jen Psaki said.

Militants are closing fast on the capital Baghdad after sweeping up a huge swath of predominantly Sunni Arab territory in northern and north-central Iraq since launching their offensive in the second city of Mosul late on Monday.

Psaki stressed however that the US embassy in Baghdad was still operating, saying "the status of the staffing at the US embassy and consulates has not changed."

The evacuation of Balad was being handled by the companies and did not involve the US government, the defence official said.

"It’s their people. It’s their planes," the official said, asking to remain anonymous.

The contractors are hired out by the Iraqi government and working on programs related to F-16 fighter jets. They are not on the US government’s payroll.

Balad airbase was once one of the world's busiest airports and housed some 36,000 American personnel before it was handed over to Iraqi control in November 2011.

The 332nd Air Expeditionary Wing was the last unit to leave JBB, which occupied 25 square kilometres and had a 20-kilometre security perimeter.

Top US weapons maker Lockheed Martin Corp on Thursday said about 25 of its employees were being evacuated from the Balad area in northern Iraq as part of a larger effort to ensure their safety given growing violence in the region.

Lockheed spokesman Michael Rein said the employees were in Iraq working with the Iraqi air force as it prepared to receive the first of 36 F-16 fighter jets, which are due to be ferried to the country later this year.

Mr Rein said it was too soon to say if the arrival of the jets in the country would be delayed as a result of the violence.

"This is an unfolding situation," Mr Rein said. "Those planes weren't due to ferry for a couple months anyway."  

Reuters, AFP