Kabul, Afghanistan (CNN) -- Several NATO and Afghan service members, including two Americans, were killed Monday when an assailant wearing an Afghan National Security Forces uniform opened fire on the group, NATO's International Security Assistance Force officials said.

The shootout in eastern Afghanistan didn't last long as coalition forces "returned fire and killed the attacker," a U.S. official told CNN.

Two Afghan army personnel also were killed, said Gen. Zahir Azimi, an Afghan Defense Ministry spokesman. A U.S. military official told CNN that at least 10 Americans were wounded as well.

The assailant fired at the victims with a truck-mounted machine gun, Azimi said, after a meeting between coalition and Afghan forces at a military base in the Jalrez district of Wardak province, about an hour west of Kabul.

A spokesman for the Taliban, the militant group that once ruled Afghanistan, said the shooter was an Afghan police officer, but it's unclear if that was the case.

A U.S. official said a variety of Afghan security forces -- including army, national police and perhaps local police -- were at a training-related meeting.

The incident appeared to be the latest "green-on-blue" attack, or strike against coalition members by people dressed in police or army uniforms. Assailants conducting similar subterfuge killed dozens of coalition troops in 2012.

On Friday, a coalition contractor in eastern Afghanistan was killed when people wearing Afghan uniforms turned their weapons against ISAF members, NATO said.

The last coalition soldier killed in a "green-on-blue" attack was a Briton, who was slain on January 7. And in the last similar fatal assault on U.S. troops, two Americans were killed October 25.

The coalition has been working to thwart such insider attacks.

Coalition soldiers are required to have a loaded weapon within reach at all times. In addition, the coalition ended training for hundreds of Afghan soldiers last year until the completion of background checks for insurgent links.

Most of the insider attacks are believed to be the result of Afghan soldiers suffering from combat or emotional stress, a Defense Department official told CNN in September after an especially deadly weekend for coalition troops.

Only about 15% of the "green-on-blue" attacks are believed to be the result of insurgent links, and about 10% come from infiltrators not affiliated with the military, the Defense Department official said.

The latest attack comes a day after Afghan President Hamid Karzai outraged the ISAF commander by contending there are "ongoing daily talks between Taliban, American and foreigners in Europe and in the Gulf states" and "that Taliban want longer presence of foreigners -- not their departure from Afghanistan."

U.S. Marine Gen. Joseph Dunford, who oversees the NATO-led force, said that's "categorically false."

"We have fought too hard over the past 12 years. We have shed too much blood over the past 12 years. We have done too much to help the Afghan Security Forces grow over the last 12 years to ever think that violence or instability would be to our advantage," Dunford said.

Newly installed U.S. Defense Secretary Chuck Hagel visited Afghanistan this weekend on his first overseas trip since his confirmation. Hagel told reporters he tried to reassure Karzai that the United States had no unilateral back-channel talks with the Taliban.2 Americans among those killed in Afghan attack