Missiles fired from drone aircraft have killed 16 militants in southern Afghanistan, the NATO-led force said Tuesday, in a rare attack in Afghanistan using the controversial unmanned planes.

A drone was launched after troops observed large numbers of insurgents preparing a strike in the southern province of Helmand's Naw Zad district on Monday, NATO's International Security Assistance Force said in a statement.

"An unmanned aerial vehicle launched one Hellfire missile killing 13 insurgents," the force said.

Three other insurgents who "took up previously used firing positions" were also killed in a separate drone strike Monday in the Nad Ali district of Helmand, the heartland of the Taliban-led insurgency, the statement added.

Coalition forces rarely use drones in their operations against the Taliban and other insurgents battling the government in Afghanistan.

Bombings by unmanned aircraft are, however, common in neighbouring Pakistan, and are a key plank of US strategy to target militants who hide out in the lawless northwest border region then cross into Afghanistan to stage attacks.

Six US strikes have hit the district of North Waziristan already this year.

The drone programme is deeply controversial in Pakistan and fuels anti-American sentiment in the Muslim nation, but US officials say they are necessary to protect the foreign troops in neighbouring Afghanistan.

There are more than 113,000 international forces based in Afghanistan fighting an increasingly deadly insurgency which is aimed at toppling the US-backed government of President Hamid Karzai.