The Taliban have been recovering ground in the south as forward operating bases have shut as part of the plan to withdraw most foreign troops by the end of 2014.

"Four... were killed by enemy forces during a partnered operation," the release by the joint command said on Sunday.

Also in southern Afghanistan, a contracted security guard shot one soldier dead in a separate incident on Saturday. A spokesman for the NATO-led force said the nationality of the security guard was under investigation.

So-called "insider attacks" on foreign soldiers by their Afghan allies escalated last year prompting coalition troops to heighten security measures and reduce interaction between foreign and Afghan forces.

The move has helped stem the number of incidents this year but also further eroded trust between foreign service members and their Afghan allies.

If Saturday's gunman turns out to have been an Afghan security guard for coalition troops, it would be the ninth insider attack this year bring the toll of foreign personnel killed to 14.

The latest deaths bring the number of US-led soldiers killed in Afghanistan this year to at least 140.

BA/BA