Secretary General Jaap de Hoop Scheffer said in an interview that "a number of buffers and filters" had been put in place to safeguard the legality of combating what he termed the nexus between the insurgency and narcotics.
"It is in accordance with international law," he said. "And if nations at a certain stage think that they would rather not participate, they will not be forced to participate."
Two weeks ago, the alliance was embroiled in controversy after General John Craddock, the NATO commander who is also chief of U.S. forces in Europe, said troops in Afghanistan would fire on individuals responsible for supplying heroin-refining laboratories with opium without the need for evidence.
In a letter to General Egon Ramms, a German who heads the NATO command center responsible for Afghanistan, Craddock said that “it was no longer necessary to produce intelligence or other evidence that each particular drug trafficker or narcotics facility in Afghanistan meets the criteria of being a military objective. ”
Ramms questioned the legality of the proposal, warning that it would violate international law and rules governing armed conflict. Ramms' letter was leaked, provoking a debate within NATO about the conditions and circumstances under which troops could attack drug laboratories.
General David McKiernen, US commander of the 55,000-strong NATO-led International Security Assistance Forces in Afghanistan, also objected to the proposal.
Hoop Scheffer ordered an investigation into the leak.
"Our enemies and opponents in Afghanistan are reading this leak," he said. "They are not stupid."
Whoever leaked the letter, "bears a heavy responsibility, responsibility for getting people killed," he said, adding: "And that I can never accept. Never. ”
NATO defense ministers, meeting in Budapest in October, agreed that their forces could destroy any drug laboratories "and facilitators" who supported the insurgency. That came after a heated debate among the 26 alliance countries.
Several members, including Germany and France, opposed the idea of NATO's taking on a counter-narcotics role, which is normally a civilian rather than a military task.
Moreover, German diplomats have feared that a more robust NATO approach towards dealing with the narcotics industry, without giving small poppy farmers an alternative livelihood, would turn more Afghans against the alliance.
Since the Budapest meeting, NATO's senior commanders have "focused on the nexus between insurgency and narcotics," Hoop Scheffer said.
The new strategy coincides with plans by General David Petraeus, the US commander responsible for Afghanistan and Pakistan, to send up to 30,000 more American troops, but also calls on NATO to supply more military equipment, aircraft, soldiers and logistics to defeat the insurgency.
Since taking office a month ago, President Barack Obama has pledged to make Afghanistan, along with the Middle East, his main foreign policy priorities.
On Monday, Richard Holbrooke, Obama's special representative to Afghanistan and Pakistan, traveled to the region with no illusions about the task facing him.
He told leaders and defense experts attending the Munich Security Conference last weekend that bringing peace to Afghanistan would be much more difficult than in Iraq, and that it would require a substantially larger effort from the international community, including NATO.