KABUL, Afghanistan (CNN) — A man alleged to be a local Afghan policeman killed a NATO service member in eastern Afghanistan on Monday, the second fatal shooting of NATO service members in a day, both apparently at the hands of their Afghan comrades.

Two British members of NATO’s International Security Assistance Force were shot and killed earlier in the southern province of Helmand by an Afghan army soldier, NATO and local officials said.

The incident happened outside the Provincial Reconstruction Team’s headquarters in the city of Lashkar Gah. 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 nationality of the third person killed Monday wasn’t immediately known. 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.

Monday’s deaths bring to 93 the number of coalition service members who have died in Afghanistan this year. It is not known how many of the incidents were attacks by Afghan security forces on ISAF troops, but a NATO analysis last year found that 52 U.S. and allied service members had been killed in such “green on blue” attacks between 2005 and June 2011.

In some attacks, insurgents have disguised themselves as Afghan soldiers in order to infiltrate bases.

Incidents of Afghan security forces turning on their international allies have fueled mutual distrust at a critical juncture of the long-running conflict.

The latest killings come after a shooting rampage in Afghanistan this month left 17 villagers dead in the Panjwai district of southern Kandahar province. A U.S. soldier, Staff Sgt. Robert Bales, has been charged with murder in the slayings.

Speaking after the deaths of the two British soldiers Monday, Gen. John Allen, the ISAF commander, said he could not discount revenge as a factor.

“I don’t connect the two of those, but in any case it is prudent for us to recognize that, as you know, revenge is an important dimension in this culture,” Allen told a briefing in Washington. “I have seen no indications yet that it has emerged as a potential factor, but we will certainly keep an eye on it.”

The killings have led to protests in the nation.

Disputes can arise from cultural misunderstanding, religious and ideological friction or combat stress, said Brig. Gen. Stephen Townsend, director of the Pakistan/Afghanistan Coordination Cell in the U.S. Joint Chiefs of Staff office.

Townsend said cultural training has been vital for U.S. soldiers, and now the Afghans are considering the same to provide a better understanding of Americans.

The Helmand governor on Monday praised ISAF troops for their sacrifices and assistance to the Afghan people.

“The enemies of the people and peace want to finish confidence among Afghan and ISAF forces, but they will never cover their evil aims by carrying out such violent acts,” he said in a statement.