Three civilian police training advisers, two American and one British, were killed over the weekend by an Afghan policeman at a training academy in western Afghanistan, Afghan officials and a NATO official said on Monday.

In a separate incident, the NATO official said two service members with the U.S.-led coalition were wounded on Monday when an Afghan soldier opened fire on them in northern Afghanistan. No other details have been disclosed about that shooting.

The official spoke on condition of anonymity because the two incidents are still under investigation.

Afghan security forces or militants dressed in their uniforms have been killing a rising number of coalition forces, but they have not been specifically targeting civilian contractors working for the coalition. So far this year, 26 foreign troops have been killed in this type of attack.

Sunday's shooting occurred at a police training academy 20 kilometers (12 miles) south of the western city of Herat.

Earlier, two Afghan police officials in Herat said that initial reports indicated that all three of the victims were Americans. The officials also spoke on condition of anonymity because they were not authorized to speak about the case.

They said two other people were wounded _ an Afghan translator and a fourth civilian adviser whose nationality is not known.

The officials said that the gunman had graduated from the police training center 1 1/2 years ago and was assigned to the center's protection unit. He was killed after he opened fire on the civilian trainers inside a hall at the training center, they said.

Last year there were a total of 21 fatal attacks by uniformed Afghans that killed 35 coalition service members, according to coalition figures. That compares with 11 fatal attacks and 20 deaths the previous year. In 2007 and 2008 there were a combined total of four attacks and four deaths.