Gen Dannatt said the campaign was a "marathon, not a sprint"
Extra British troops may be sent to Afghanistan, the head of the Army, General Sir Richard Dannatt, has said.
Sir Richard told the Times part of 12 Mechanised Brigade - who had been training for deployment to Iraq - was now "earmarked" for Afghanistan.
The Ministry of Defence said there was no decision on the number or timeline.
Meanwhile the US is planning to send an extra 4,000 troops to help train Afghan security forces, on top of 17,000 reinforcements already due to arrive.
The extra troops are expected to arrive in June, according to US Senator Carl Levin.
The Times said Downing Street had taken part in discussions about a British surge, ahead of an expected requested from the US for more troops.
Gen Dannatt said there were no plans to send the whole brigade of 4,000 troops.
He said the new level of troops would lie "somewhere in between" the current 8,000 and the 12,000-strong presence the UK would have if the whole brigade was sent.
Gen Dannatt said sending a whole brigade would put too much strain on Army resources.
"If we were to send another 4,000... there would be a risk of replicating the pressures on the Army that we are trying to avoid," he told the paper.
Afghanistan was going to be a "marathon campaign, not a sprint," he added, saying that members of the forces needed time off.
This week the new UK ambassador to Afghanistan said British forces may be in the country for another five years while civilians could stay more than 20 years.
Mark Sedwill, whose job will involve close liaison with the British military command, said the Afghan army and police should be strong enough in five years to take over.
British workers would be there much longer helping with reconstruction and development, he added.
Some of the new US troops already announced will be deployed in Helmand province, where British and Taleban forces have been involved in fierce fighting.
President Barack Obama is expected to announce the latest extra commitment on Friday when he outlines his new Afghanistan strategy.