KABUL, Afghanistan (AP) - Officials say an Afghan police officer has shot and killed three U.S. Marines in southern Afghanistan. It is the third attack on coalition forces by their Afghan counterparts in a week.
   
A U.S. Defense Department official confirmed Friday that the dead Americans were Marine Special Operations Forces. He spoke on condition of anonymity because the family notification process was not complete.
   
Thirty coalition service members have now died this year at the hands of Afghan forces or insurgents disguised in Afghan uniforms, according to an Associated Press tally - a dramatic rise from previous years.
   
The assaults have cast a shadow of mistrust over U.S. efforts to train Afghan soldiers and police more than 10 years after the U.S.-led invasion to topple the Taliban's regime for sheltering the al-Qaida leadership.

(Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)

Earlier story:

KABUL, Afghanistan (AP) - A man in an Afghan army uniform shot and killed three American service members this morning in southern Afghanistan.
   
The U.S. military command says the shooting happened in Helmand province, but is not giving additional details.
   
The attack is the third killing this week of coalition soldiers by Afghans they are training to take over responsibility for security once most international forces leave in 2014.

(Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)