British troops will not be pulled out of Iraq to cover a new deployment in Afghanistan, Defence Secretary John Reid has said.
Mr Reid is touring the region
Nato forces there come under UK control in May, but Mr Reid said claims 4,000 UK troops would be sent were wrong.
He said: "We do not need to reduce forces in Iraq to supply the necessary configuration in Afghanistan."
Nato troops are due to move into the south next year, which has become a stronghold for militants.
British troops will also support efforts to take on the armed drug gangs operating in the region.
Mr Reid said they would offer backing to local forces charged with dismantling the booming economy based on drug sales.
It is estimated 90% of all heroin in Britain orginates from Afghanistan.
Britain has so far committed itself to deploying an extra 400 military personnel in Afghanistan to staff the Allied Rapid Reaction Corps, which it takes over from the Netherlands in May.
Mr Reid, who is currently touring the region, told BBC News he had not yet decided how many UK troops would be needed.
He said a decision would be taken "once the generals and the military and politicians in Europe have decided how we need to go into the south".
He dismissed criticism that the government is asking the armed forces to do more with less investment and resources.
"We are asking our forces to do more than was previously the case and we are trying to supply them with the capabilities that are necessary," he said.
Differences of opinion
The US has been pushing for Nato, which has 12,800 troops in the country, to take a more offensive role to ease the pressure on its own fight against militants who operate mainly in the south and east.
But France, Germany and Spain believe Nato troops should keep to peacekeeping.
Earlier in his trip, Mr Reid said these differences would have to be "worked through".
The US has about 18,000 soldiers in Afghanistan and has lost more than 50 troops this year, making it the worst year for its military fatalities since the fall of the Taleban in December 2001.