KABUL, Afghanistan – A veteran Afghan military officer gunned down six NATO service members and wounded five Afghan soldiers Wednesday after an argument at the Kabul airport, Afghan and coalition officials said.

It was the seventh incident so far this year in which members of the Afghan security forces, or insurgents impersonating them, have killed coalition soldiers or members of the Afghan security forces.

NATO did not disclose the nationalities of the troops killed pending notification of their families.

The Taliban claimed responsibility for the attack, but Defense Ministry spokesman Gen. Mohammad Zahir Azimi said the gunman was an Afghan military pilot who "opened fire on foreigners after an argument."

The pilot was killed in the morning shooting, which occurred at a facility used by the Afghan Air Force, Azimi said.

Azimi said the shooter was a military pilot of 20 years. "An argument happened between him and the foreigners and we have to investigate that."

An Afghan pilot who spoke on condition of anonymity, said the gunman was Ahmad Gul, a 50-year-old pilot from Tarakhail district of Kabul province.

In a statement, Taliban spokesman Zabiullah Mujahid said, however, that the gunman was impersonating an army officer and that others at the facility helped him gain access. The gunman killed nine foreigners and five Afghan soldiers, he said. The Taliban often exaggerate the number of casualties caused by their attacks.

Taliban insurgents have stepped up their attacks on government and military installations across Afghanistan.

_On April 18, an insurgent managed to sneak past security at the heavily fortified Afghan Defense Ministry compound in the capital and killed two Afghan soldiers and an officer.

_Two days before that, an Afghan soldier walked into a meeting of NATO trainers and Afghan troops at Forward Operating Base Gamberi in Laghman province in eastern Afghanistan and detonated a vest of explosives hidden underneath his uniform. The blast killed six NATO troops, four Afghan soldiers and an interpreter.

_On April, 15, a suicide bomber dressed as a policeman blew himself up inside the Kandahar police headquarters complex, killing the top law enforcement officer in the restive southern province.

_In northwest Afghanistan, a man wearing an Afghan border police uniform shot and killed two American military personnel on April 4 in Faryab. The gunman was upset over the recent burning of the Quran at a Florida church, according to NATO intelligence officials.

_In February, an Afghan soldier, who felt he had been personally offended by his German partners, shot and killed three German soldiers and wounded six others in the northern province of Baghlan.

_In January, an Afghan solider killed an Italian soldier and wounded another in Badghis province. The two soldiers were cleaning their weapons at a combat outpost when an Afghan soldier approached them with an M16 rifle and asked to use their equipment to clean his gun. The Italians saw that the Afghan soldier's rifle was loaded and asked him to unload it, at which point the Afghan soldier shot the two Italians and escaped from the base.

Before Wednesday's shooting at the airport, the coalition had recorded 20 incidents since March 2009 where a member of the Afghan security forces or someone wearing a uniform used by them attacked coalition forces, killing a total of 36. It is not known how many of the 282,000 members of the Afghan security forces have been killed in these type of incidents.

According to information compiled by NATO, half of the 20 incidents involved the impersonation of an Afghan policeman or soldier. The cause of the other 10 incidents were attributed to combat stress or unknown reasons. NATO said that so far, there is no solid evidence — despite Taliban assertions — that any insurgent has joined the Afghan security forces for the sole purpose of conducting attacks on coalition or Afghan forces.