Reuters reports: Four U.S. troops fighting with the NATO-led alliance were killed in another suspected “insider” attack in southern Afghanistan on Sunday, bringing the total number of deaths this weekend caused by Afghans turning on their allies to six.

Four troops were found dead and two wounded when a response team arrived at the scene from a nearby checkpoint, a spokesman for the coalition said. A Pentagon spokesman confirmed that the four dead were Americans.

One of the six members of the Afghan National Police (ANP) operating the observation post with six coalition troops was also found dead, while the other five had disappeared.

“The fighting had stopped by the time the responders arrived,” said Major Adam Wojack, a spokesman for the NATO-led coalition.

Sunday’s shooting took place in Zabol, a province where U.S. forces are based, according to a local official.

The attack came a day after two British soldiers were shot dead by an Afghan policeman while returning from a patrol in the southerly Helmand province, one of the strongholds of the Taliban-led insurgency.

At least 51 foreign military personnel have been killed in “insider” attacks this year, deaths that have badly strained the coalition’s relations with Afghan forces as it moves towards handing security responsibility to them by the end of 2014.