NATO has said it has scaled back operations with members of the Afghan National Security Forces in an attempt to lower the risk of so-called insider attacks.

A total of 51 international troops have been killed by Afghans in the uniforms of the nation's police and military forces so far this year.

Until recently, elements of NATO companies numbering roughly 100 soldiers routinely conducted operations like patrolling or manning an outpost with Afghan soldiers.

NATO said such operations are no longer routine and require the approval of the regional commander.

Al Jazeera's Rosiland Jordan, reporting from Washington, said a Pentagon official had told Al Jazeera that General John Allen, commander of all international forces in Afghanistan, "has commanders to take a top-to-bottom look at how coalition and Afghan forces are paired across the country".

Jordan said the re-assessment of the pairing is meant to "reduce the opportunity for these [foreign] troops to be injured or killed" while serving with their Afghan counterparts.

The directive to scale back partnership with Afghan forces was issued by Lieutenant General James Terry on Sunday.

The latest "insider attack" occurred on Sunday when several Afghan men in police uniform killed four US soldiers and wounded two others at a checkpoint in the Mizan district of Zabul province.

The Zabul attack was preceded by an attack on a military base in Helmand province on Friday.

The attack at Camp Bastion, which the Taliban has claimed responsibility for, left two US soldiers dead and destroyed six attack jets.

Though Camp Bastion is considered one of the largest and best-defended posts in Afghanistan, the attack was the single most destructive strike on Western armaments in the 11-year-long war.