The US envoy to Nato has said that a British-led military force due to move into southern Afghanistan must be ready to fight resurgent Taleban militants.
Nato troops will have kept out of the south until now
Ambassador Victoria Nuland said Nato would need to provide "a strong and robust fighting force" in the region.
Scores have died in a spate of attacks in the south in recent months, among them a Canadian diplomat on Sunday.
Nato has about 9,000 peacekeepers in Afghanistan and will expand its role to the south where US-led troops operate.
BBC defence correspondent Paul Wood says British troops are going to the south to guarantee reconstruction, not to engage in American-style capture and kill missions.
But Ambassador Nuland said there should be no illusions about the "rigorous environment" in the south.
"Nato forces, in providing security and stability throughout the country except to the east, will be prepared to perform missions up to and including what we call counter-insurgency, which obviously will require a strong and robust fighting force," she said.
The escalation of violence in southern Afghanistan has led the Dutch government to delay a decision on whether to send more than 1,000 troops to join the Nato mission.
15 January 2006: Canadian diplomat and 2 Afghans killed in Kandahar
5 January 2006: 10 killed in Tirin Kowt, Uruzgan
November 2005: 3 civilians killed in Kandahar
September 2005: 12 killed outside Afghan army base in Kabul
May 2005: 3 killed in Kabul internet cafe
June 2005: 20 killed in Kandahar mosque
But our correspondent says he understands the British government has decided it cannot wait any longer before making an announcement on the number of troops it will provide.
He says the Dutch position does not mean the UK will deploy more troops, but it is a setback for British hopes that Nato can quickly take over from the US in Afghanistan.
Meanwhile, violence in the south is on the rise.
There have been at least three suspected suicide bombings in the southern province of Kandahar since the weekend, and Taleban commanders are threatening to carry out more attacks.
One blast on Monday killed 20 Afghans in Spin Boldak near the Pakistan border. A day earlier a senior Canadian diplomat was among three killed in another bombing in the city of Kandahar itself.
Last year was the deadliest in Afghanistan since 2001, with more than 1,400 people killed in violence linked to militancy, most of them in the south and east.
Much of the violence has been blamed on remnants of the hardline Taleban movement, which governed Afghanistan until the US-led invasion four years ago.