KABUL, Afghanistan –  A man in an Afghan police uniform shot and killed two American service members in what appeared to be the latest attack 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.

Before Thursday's assault, 53 foreigners attached to the U.S.-led coalition had been killed in insider attacks from Afghan soldiers or police so far this year, according to the NATO military coalition.

The Taliban have said they are using the attacks as a specific strategy to drive a wedge between the international coalition and the Afghans. On Wednesday, ahead of the Eid al-Adha holiday, they called for a boosting of such assaults.

"Increase your efforts to expand the area of infiltration in the ranks of the enemy," Mullah Mohammad Omar urged in a statement. "Jihadist activities inside the circle of the state militias are the most effective stratagem."

The majority of international combat troops are scheduled to leave Afghanistan by end-2014, with trainers and advisers expected to remain longer. But the spate of insider attacks has further undermined public support for the 11-year war in NATO countries and increased calls for earlier withdrawals.

The attacks have not been limited to members of the NATO-led international coalition. More than 50 Afghan members of the government's security forces also have died this in attacks by their own colleagues.

Also Thursday, an Italian soldier was killed in a firefight in Farah province in the west of the country, the Italian military said in a statement. Three other soldiers were wounded.

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Associated Press writer Mirwais Khan in Kandahar contributed to this report.