NATO said on Sunday it had intercepted a letter from fugitive Taliban leader Mullah Mohammad Omar in which he called for any Afghan supporting their country's government to be captured or killed.
Omar had issued the directive in June, NATO spokesman Brigadier Josef Blotz said, adding that the Taliban chief was believed to be in hiding in neighbouring Pakistan.
"The message was from Mullah Omar, who is hiding in Pakistan, to his subordinate commanders in Afghanistan," Blotz said.
He said the order to Taliban fighters was to fight coalition forces to the death, and to capture and kill any Afghan civilian supporting or working for coalition forces or the Afghan government.
It also encouraged the recruitment of any Afghan with access to NATO or US bases in the country, Blotz told reporters.
The one-eyed Omar is a founder of the Taliban and is often referred to as its "supreme commander" or spiritual leader. Many analysts and diplomats have long believed he is in Pakistan, although Islamabad has denied his presence.
The letter, if genuine, appears to be a departure from an earlier directive that urged Taliban not to harm captives.
"Whenever any official, soldier, contractor or worker of the slave government is captured, these prisoners cannot be attacked or harmed," said the August 2009 code of conduct, attributed to Omar.
By contrast, the latest letter says women should also be killed if found to be helping or providing information to coalition forces.
The United States and NATO have almost 150,000 troops deployed to Afghanistan, fighting an insurgency of almost nine years that is becoming more virulent as foreign forces take the fight to the Taliban's heartland.
Casualties among foreign troops have spiked in recent months, with more than 370 killed so far this year, compared to 520 for all of 2009.
Military commanders say the higher death toll was expected as battlefield engagements are escalating with the coalition's attempts to speed an end to the war.