KABUL, Afghanistan – An Afghan police officer shot and killed three U.S. Marines after sharing a meal with them before dawn Friday and then fled into the desolate darkness of southern Afghanistan, the third attack on coalition forces by their Afghan counterparts in a week.

Thirty-one coalition service members have now died this year at the hands of Afghan forces or insurgents disguised in Afghan uniforms, according to NATO – a dramatic rise from previous years.

The assaults have cast a shadow of fear and 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 hard-line Islamist regime for sheltering al-Qaida’s leadership.

The attacks also raise further doubts about the quality of the Afghan forces taking over in many areas before most international troops leave the country in 2014.

Friday’s deadly shooting took place in the volatile Sangin district of Helmand province, U.S. military spokeswoman Maj. Lori Hodge said. Sangin was a Taliban stronghold for years and has one of the highest concentrations of improvised explosive devices, or IEDs, in the country.

A U.S. Defense Department official confirmed that the dead Americans were Marine Special Operations Forces. He spoke on condition of anonymity because the family notification process was not complete.

Sangin’s district chief and the Taliban both identified the gunman as Asadullah, a member of the Afghan National Police who was helping the Marines train the Afghan Local Police.

Taliban spokesman Qari Yousef Ahmadi said the attacker joined the insurgency after the shooting. “Now, he is with us,” Ahmadi said.

The district chief, Mohammad Sharif, said the shooting occurred at a police checkpoint after a joint meal and a security meeting. The meal took place before dawn because of Ramadan, the month in which Muslims abstain from food during daylight.

Compared with the 25 attacks this year that have killed 31 foreign troops, there were 11 such attacks and 20 deaths in 2011, according to an Associated Press count.

Each of the previous two years saw five such attacks.

A spokesman said President Obama remains committed to his timeline to transfer the security lead to Afghan forces by the end of 2013.