Nato countries France, Germany and Spain have told the US the organisation should keep to its peacekeeping tasks in Afghanistan.
Nato peacekeepers operate mainly in the north and west
They said at a meeting in Berlin Nato should not become involved in offensive operations to target the Taleban.
US Defence Secretary Donald Rumsfeld had appealed for more Nato help in tackling the insurgency.
There are more than 12,000 Nato peacekeepers in Afghanistan and about 18,000 US troops.
France and Spain said the Nato peacekeepers should remain separate from the US-led Operation Enduring Freedom, which mainly operates in the south and east.
Nato took over command of the International Security Assistance Force (Isaf) in Afghanistan in August 2003. Most Nato troops operate in the north and west.
"These missions must remain separate with separate chains of command. The only thing they have in common is that they are in the same country," said Spanish Defence Minister Jose Bono Martinez.
German Defence Minister Peter Struck said: "I would not like to expose our soldiers to more danger by linking these two mandates together."
Britain, Canada and the Netherlands have pledged soldiers for an expanded Nato operation in the south.
Mr Rumsfeld had said it would be "nice if Nato developed counter-terrorist capabilities".
He vowed the US would "continue to play a strong role" despite speculation the US was planning to cut its contingent.
The New York Times and Washington Post reported the cuts could be up to 20% by early 2006.
Security remains a key concern for Afghanistan, with US forces suffering their heaviest losses this year since the 2001 invasion.