Defence Secretary John Reid has said there may be occasions where British troops are used to seek out and kill Taleban and al-Qaeda terrorists.
Mr Reid spent Sunday meeting politicians in Kabul
But on a visit to meet British forces in Afghanistan he stressed their main job was to help reconstruction efforts.
Mr Reid spent Sunday with politicians in the capital Kabul before travelling to volatile Helmand in the south, where 3,300 UK soldiers are being deployed.
He has warned terrorists are out to destroy rebuilding efforts.
"Although our mission to Afghanistan is primarily reconstruction, it is a complex and dangerous mission because the terrorists will want to destroy the economy and the legitimate trade and the government that we are helping to build up," he told the BBC from Kandahar airbase in Afghanistan.
"Of course, our mission is not counter-terrorism but one of the tasks that we may have to accomplish in order to achieve our strategic mission will be to defend our own troops and the people we are here to defend and to pre-empt, on occasion, terrorist attacks on us.
"If this didn't involve the necessity to use force we wouldn't send soldiers."
He said UK forces would be providing security for the reconstruction of Helmand, which could take three years.
He has denied being asked to provide extra troops for the mission.
On Sunday, Mr Reid met Afghan defence minister Rahim Wardak to discuss next month's deployment of British troops.
At a press conference he denied the number was not enough and said the task was to "help and protect" the Afghan people's reconstruction aim.
British troops are to take over the Provincial Reconstruction Team (PRT) from the US forces in Helmand.
Mr Reid told reporters it was "absolutely and completely untrue" that he had received requests from Army commanders for 600 more troops.
"Just in case there had been a request from any quarter which I had not yet received, I clarified the position this morning with the commander of British forces here."
He said the role of the British forces in Helmand was fundamentally different to that of the US forces elsewhere in Afghanistan.
He said: "We are in the south to help and protect the Afghan people construct their own democracy.
"We would be perfectly happy to leave in three years and without firing one shot because our job is to protect the reconstruction."
The American mission was to "go and chase and kill the terrorists who did so much to destroy the twin towers in that terrible attack," he said.
However, he clarified that "nobody should be under any illusion that if attacked we will defend ourselves" and "respond in a way that defends our troops".
The British mission is to help the Afghan people with reconstruction
Britain originally stationed 1,100 soldiers in Afghanistan, but has begun sending extra troops to Helmand province, which has seen outbreaks of Taleban violence.
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.
Coalition troops must maintain the offensive against Taleban and al-Qaeda militants in Afghanistan to prevent their return to power, Mr Reid said.
"The greatest danger of all for the people of Afghanistan and the people of the United Kingdom would be if Afghanistan ever again came under the rule of a Taleban regime prepared to protect al-Qaeda or terrorist groups," he said.
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.
Isaf currently numbers about 9,200 troops, but is expected to increase to about 15,000.
The UK takes control of Nato forces in Afghanistan in May.