(CNN) -- An Afghan soldier killed two American troops Monday in the latest attack by Afghan security forces on coalition soldiers, authorities said.
The killings occurred the same day 10 Afghan soldiers were killed when members of their own unit opened fire on them at an army post in southern Helmand province, according to provincial officials.
The latest attacks come at a sensitive time for the United States and its NATO-led allies, who are preparing to withdraw the majority of their troops by 2014 under a plan that requires Afghan forces to take responsibility for the country's security.
A defense official not authorized to speak on the record confirmed the nationality of the two Americans. The official said circumstances are still under investigation, but it appears to be an insider attack.
With Monday's killing of the two, at least 42 NATO troops have been killed in attacks by Afghan forces or insurgents disguised as soldiers or police, according to coalition officials.
U.S. Marine Gen. John Allen, chief of the NATO-led International Security Assistance Force, said last week that an estimated 25% of such attacks are carried out by Taliban infiltrators.
The coalition did not provide details of the attack in eastern Afghanistan, saying only that NATO "troops returned fire, killing the ... soldier who committed the attack."
Afghan authorities, meanwhile, are investigating the attack by five soldiers on their own unit in Helmand's Washir district, which killed 10 people and wounded four, said Daud Ahmadi, a spokesman for Helmand's provincial governor.
There has been no claim of responsibility for the killings, and the governor's spokesman did not outline a possible motive for the attack.
Elsewhere in Afghanistan, authorities accused the Taliban of killing 17 people, including two women, in the Shah Karez region of Helmand province north of Kandahar, the Interior Ministry said Monday. A dispute between two Taliban commanders over two women led to the killings, Ahmadi said. Earlier reports by the Interior Ministry that said the victims were beheaded were incorrect, according to Ahmadi, who said the 17 were either shot or stabbed.
The attack occurred Sunday, the ministry said. The provincial government confirmed the killings but stopped short of accusing the Taliban, saying officials were still trying to determine who was responsible.
Allen condemned the latest violence, calling those responsible for it "cowards, killing innocent men and women."
CNN's Chelsea J. Carter and Barbara Starr contributed to this report.