Taleban fighters have told the BBC they plan to target and kill British troops starting a tour of duty in Afghanistan.
Taleban fighters say they are skilled at guerilla warfare
A local Taleban commander in Helmand, one of the country's most volatile provinces, called the British "an old enemy of Afghanistan".
The comments emerged as Defence Secretary John Reid visited UK troops in the southern Helmand province, where they are aiding reconstruction efforts.
He acknowledged the 3,300 soldiers being deployed faced "massive risks".
He stressed their main job was to help reconstruction efforts, but said they may be used at times to seek out and kill Taleban and al-Qaeda terrorists to prevent their return to power in Afghanistan.
Violence has been increasing in recent weeks, with a series of roadside bombings against security forces.
British officers in Helmand have said they are facing a "rocky period ahead".
But Brigadier Ed Butler, commander of British forces in Afghanistan, said there was a chance to bring lasting security to the country.
He said: "We need to expect some setbacks and we need to prepare ourselves and the public.
"But we genuinely think we can make a difference and there is a window of opportunity here to improve the life of the ordinary Afghan.
"We will do that by providing him with security and they have not had that security for the last 30 years. We are realistic that is going to take time."
British troops in Helmand are thought to be most at risk from roadside bombings and attacks by suicide car bombers.
Insurgents have been distributing DVDs on the Pakistan border trying to recruit locals in Afghanistan for attacks.
Taleban fighters on the ground in Helmand are specifically threatening British troops.
One man, a local Taleban commander, told the BBC: "The British have been defeated in the past. Afghans are not scared of death.
"The British are an old enemy of Afghanistan. Our resources are getting better day-by-day and we have good skills of fighting guerrilla war."
British troops are taking over the Provincial Reconstruction Team (PRT) from US forces in Helmand.
They hope to provide the security to help efforts to stop opium production and promote rural development, British officials say.
Mr Reid spent Sunday with politicians in the Afghan capital Kabul before travelling to Helmand.
Speaking during the visit to the base, Mr Reid warned terrorists were out to destroy rebuilding efforts.
He said "nobody should be under any illusion that if attacked we will defend ourselves" and "respond in a way that defends our troops".
At present about 2,000 British soldiers are based in Afghanistan as part of the Nato-led International Security Assistance Force (Isaf) and US-led coalition forces.
The Army is preparing to deploy the full task force of 3,300 troops, led by 16 Air Assault Brigade, to start operations in June.