The shooter was a member of the Afghan special forces, military spokesman Mohammad Rasoul Zazai told the Reuters news agency on Sunday.

The US army also confirmed that three Americans were wounded at the Camp Shorab air base in the south of the country.

"Coalition security forces on the base killed the soldier to end the attack," a spokesman for US forces told Agence France-Presse (AFP). "The US soldiers are receiving medical treatment at this time, and we will release more information when available."

Settling grudges

Gunfights between coalition troops and local soldiers are not uncommon in Afghanistan. Such incidents, also known as "insider attacks," have claimed the lives of more than 150 coalition troops since 2008, according to the US Long War Journal website.

While undercover Taliban militants commit some of the "green on blue" shootings, many cases are sparked by personal grudges or a perceived insult between the locals and the foreign soldiers. Such incidents have proven extremely toxic for the cooperation between the allied troops.

The shootings have dwindled after peaking in 2012, when over 60 foreign troops lost their lives. Sunday's incident in southern Helmand province marks the first insider attack since the beginning of the year.

In May last year, attackers in Afghan military uniforms killed two Romanian soldiers in neighboring Kandahar province.

dj/tj (AFP, AP, Reuters)