A man dressed as an Afghan border police officer opened fire on NATO troops during a training mission in the east of the country Monday, killing six Americans before he was shot dead, NATO and Afghan officials said.

A U.S. official who spoke on condition of anonymity said the victims were Americans.  The official did not provide additional details.

The shooting -- the highest toll for NATO forces since nine Americans died in a Sept. 21 helicopter crash -- was the latest in a series of shootouts in which Afghan security forces have turned on their NATO partners.

The attack also highlights the potential hazards of a push to speedily expand Afghanistan's army and police forces in the next few years. The goal is to turn over the responsibility for nationwide security to Afghan forces by 2014 so that NATO troops can go home.

The shooter was wearing an Afghan border police uniform, NATO said, but did not provide additional details on how the shooting happened or his identity. A spokesman for the Afghan Interior Ministry, Zemeri Bashary, confirmed that the gunman was a border police officer, rather than an insurgent who had donned the uniform to infiltrate government forces.

The incident happened in Pachir Wagam district of Nangarhar province -- right on the border with Pakistan, Bashary said.

NATO declined to identify the nationalities of the victims. The majority of forces in Nangarhar are American.

An investigation team has been sent to the area, said Gen. Aminullah Amerkhail, the regional border police commander for eastern Afghanistan. But he said information is not coming back quickly.

"The area is very remote," he said. "Even the telephones are not working there."

There have been a number of incidents in which Afghan police officers turned on their trainers in deadly shootouts.

NATO is investigating an incident in which two U.S. Marines were killed earlier this month in southern Helmand province, allegedly at the hands of an Afghan army soldier.

On July 20, an Afghan army sergeant got into an argument at a shooting range in northern Afghanistan and shot dead two American civilian trainers before being killed. Another Afghan soldier was killed in the crossfire. In a July 13 attack, an Afghan soldier stationed in the south killed three British troopers, including the company commander, with gunfire and a rocket-propelled grenade in the middle of the night.

Also, in November 2009, an Afghan policeman killed five British soldiers at a checkpoint in Helmand. A month earlier, an Afghan policeman on patrol with U.S. soldiers fired on the Americans, killing two.

Such shootings highlight the potential hazards of a push to speedily expand Afghanistan's army and police forces in the next few years. The goal is to turn over the responsibility for nationwide security to Afghan forces by 2014 so that NATO troops can go home.

In the past year, the Afghan police force grew 27 percent to 120,500 officers from about 95,000. The army grew 42 percent to about 138,200 soldiers from 97,000.

The Associated Press contributed to this report.