KABUL, Afghanistan (AP) — A man in an Afghan police uniform shot and killed two American service members Thursday in what appeared to be the latest in a rash of attacks on international forces this year by their Afghan partners.

The so-called insider attacks have stretched to the breaking point a partnership that U.S. and NATO officials consider a key part their exit plan — preparing the Afghans to take over responsibility for their country’s security in just over two years’ time. They have also cast major doubts over the program, where Afghans and international troops are supposed to work “shoulder to shoulder.”

In Thursday’s shooting, authorities had yet to determine if the attacker was an Afghan police officer or an insurgent who had donned a uniform to get close to the Americans, said Maj. Lori Hodge, a spokeswoman for U.S. forces in Afghanistan. The assailant escaped after killing the service members while they were out on a late morning patrol in the southern Uruzgan province, she added.

It was the second suspected insider attack in two days. On Wednesday, two British service members and an Afghan police officer were killed in an “exchange of gunfire” in Helmand province, the British Ministry of Defense said in a statement. The Afghan officer was not wearing his uniform and the statement said it was not clear who started shooting first

Provincial government spokesman Abdullah Himmat said the assault happened near an Afghan police compound in Khas Uruzgan district.