A newly recruited Afghan village policeman opened fire on his American allies on Friday, killing two U.S. service members minutes after they gave him a new weapon as a present. It was the latest in a disturbing string of attacks by Afghan security forces on the international troops training them.

Later Friday, another Afghan in uniform turned his gun on foreign troops in another part of the country, but no one was killed in the second attack, a spokesman for the NATO-led coalition force said.

The attacks in the country's far west and south brought to seven the number of times that a member of the Afghan security forces -- or someone wearing their uniform -- has opened fire on international forces in the past two weeks.

Such assaults by allies, virtually unheard of just a few years ago, have recently escalated, killing at least 36 foreign troops so far this year. They also raise questions about the strategy to train Afghan national police and soldiers to take over security and fight insurgents after most foreign troops leave the country by the end of 2014.