Afghanistan's defence ministry has expressed concern over the slow pace of developing its airforce ahead of a scheduled withdrawal of NATO troops and equipment, the government said Sunday.

More than a dozen transport aircraft provided to the Afghan airforce by the United States have been grounded because of age, a lack of spares and safety problems, President Hamid Karzai's office said in a statement.

The issue was raised at a security meeting which was told that the defence ministry was "concerned over the slow pace of reviving the country's air force" and wanted the US to "intensify its efforts for that end".

As part of its exit strategy from the Afghan war, the US is helping Kabul build its airforce before most air support from NATO forces is withdrawn along with 130,000 troops by the end of 2014.

Fifteen C-27 transport aircraft supposed to provide support to the Afghan army and deliver humanitarian aid had been grounded for two years, defence ministry spokesman Zahir Azimi told AFP.

"The planes were made in Italy, they are old and were out of the Italian fleet before they were given to Afghanistan," Azimi said.

Air transport is critical in Afghanistan, where the road network is underdeveloped and targeted by insurgent bombings, while strike aircraft are a vital part of the war against Taliban insurgents.

The US Air Force announced last month that it was reopening a contest for a contract to build 20 light attack aircraft for Afghanistan after the cancellation of an award to Brazil's Embraer two months earlier.

But a final decision for the contract will not be made before early 2013, the Air Force said in a statement, with the first planes due to be delivered in the second half of 2014.

The new schedule will mean "a delay of about 15 months" from original plans, before the Air Force called off the award, the statement said.

Last year, an Afghan air force officer shot dead nine US service members at a training centre at Kabul International Airport, one of the deadliest attacks on coalition troops in recent years.

It was one of an increasing number of attacks in which Afghans being trained by NATO troops have turned their weapons on their mentors.