A roadside bomb has killed four policemen in Afghanistan, while NATO says at least two members of its international peacekeeping force have died.

A spokesman for the Badghis provincial governor said Saturday that the four police officers were killed when an explosive ripped through their patrol vehicle.

And NATO announced Saturday that at least one of its service members in southern Afghanistan has died in a bomb blast, while another died of non-battle-related causes. NATO does not release details about slain service members, including their nationalities.

On Friday in eastern Afghanistan, an attacker wearing an Afghan army uniform shot and killed a U.S. service member in Kunar province.

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

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.

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.