An attacker wearing an Afghan army uniform has shot and killed a U.S. service member in eastern Afghanistan.

Friday's shooting is at least the 15th incident this year in which an Afghan soldier or an insurgent disguised as a soldier has fired on coalition forces, AP reported.

NATO said it was investigating and gave few other details, but Afghan officials said the incident took place in Kunar province. The Pentagon later said the service member was American.

The Taliban claimed responsibility for the attack, though the insurgent group regularly takes credit for violence in the country even it if is not involved.

Earlier this month, NATO said coalition forces shot and killed a man dressed in an Afghan army uniform after the man fired on a group of coalition soldiers, leaving one dead. And in April, an Afghan soldier shot and killed an American service member in southern Kandahar province.NATO officials say there were at least 15 attacks in Afghanistan this year in which an Afghan soldier or insurgent disguised as one fired on coalition forces.

Here are some of those attacks:May 11: Man in Afghan army uniform kills one NATO soldier in eastern Afghanistan.

May 5: Man in Afghan army uniform kills one NATO soldier in southern Afghanistan.

April 26: Afghan soldier kills a U.S. soldier in Kandahar province.

March 26: Man in Afghan army uniform kills two coalition soldiers in southern Afghanistan, two weeks after a U.S. soldier allegedly killed 17 Afghan civilians in a neighboring province.

February 20: Reports emerge that coalition soldiers improperly disposed of Qurans. This led to several attacks by gunmen in Afghan security uniforms, killing six NATO service members.

January 8: Afghan soldier kills American counterparts in southern Afghanistan.

U.S. officials have said that many of the so-called "insider attacks" are motivated by personal disputes.

The incidents have raised concerns about security in Afghanistan at a time when international combat troops are preparing to withdraw from the country.

International forces are set to transfer full security responsibility to Afghan forces by a 2014 deadline.

Meanwhile in southern Afghanistan, NATO says an insurgent attack killed one of its service members on Friday. No other details were given.

Also Friday, the United Nations' special representative to Afghanistan, Jan Kubis, told reporters that civilian casualties have gone down by 20 percent in the first four months of this year compared to the same period in 2011. Kubis would not give exact figures, but said the majority of civilian deaths are caused by insurgent attacks, including suicide bombings.

On Thursday, a roadside bomb killed seven Afghan civilians, mostly women and children, in Helmand province. NATO on Friday condemned the attack "in strongest possible terms." The coalition says the Taliban is responsible for at least 450 deaths so far this year.

The U.N. envoy also said that there is a stronger effort by international forces to prevent civilian casualties during coalition operations. The issue has been a source of tension between the Afghan government and NATO.

On Friday, NATO said preliminary investigations found a number of Afghan civilians were killed or wounded during coalition airstrikes on May 4 and May 6 in the southern province of Helmand. In a statement, the coalition took "full responsibility for these tragic and regrettable incidents" and said it would meet with victims' family members to express condolences.

NATO said the commander of U.S. and NATO forces in Afghanistan, U.S. General John Allen, will brief Afghan President Hamid Karzai on the results of the initial probe and pledge all appropriate actions to prevent such incidents from occurring in the future. The coalition also said action will be taken to hold those responsible accountable.

Coalition spokesman, General Carsten Jacobson, said Friday that the primary task of the international force is to protect the civilian population.

In another development, Germany says President Karzai and German Chancellor Angela Merkel will discuss the withdrawal of German troops from Afghanistan next week.

A spokesman for Chancellor Merkel announced Friday the two leaders will meet in Berlin on Wednesday for wide-ranging talks ahead of NATO summit in the U.S. later this month.

Germany has the third largest contingent of soldiers serving in the NATO mission in Afghanistan.