Kabul, Afghanistan (CNN) -- A man alleged to be a local Afghan policeman killed an American service member in eastern Afghanistan on Monday, while two British troops were shot and killed by an Afghan soldier in the southern province of Helmand, according to U.S., NATO and Afghan officials. The British troops were killed outside a Provincial Reconstruction Team's headquarters in the southern city of Lashkar Gah, NATO and local officials said. Coalition forces then fatally shot the gunman. A spokesman for the provincial governor said the incident stemmed from an argument the soldier had with the victims. The eastern Afghanistan victim was an American soldier, a senior U.S. defense official, speaking on condition of anonymity, told CNN. NATO said he was shot as he approached an Afghan local police checkpoint; it said Afghan and coalition forces are investigating the incident but released no further details.
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NEW: ISAF commander says attacks have caused "an erosion of trust"
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Three NATO troops were killed Monday by what authorities say were Afghan forces
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The killings come after U.S. soldier was charged with shooting rampage this month
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The ISAF commander says he can't discount revenge as a motive
Monday's deaths bring to 93 the number of coalition service members who have died in Afghanistan this year, 44 of them @placeholder.
"The enemies of the people and peace want to finish confidence among Afghan and @placeholder forces, but they will never cover their evil aims by carrying out such violent acts," he said in a statement.