British troops in Afghanistan may have to take ground gained this summer again next year, the Nato chief has warned.
Gen McNeill said he expected the Taleban to regroup over winter
Gen Dan McNeill said the alliance had made important military gains over the past six months in Helmand.
But he warned Afghan security forces might not be able to retain security as the Taleban regroup over winter.
The Ministry of Defence said it had been increasing British forces in Helmand, which would help consolidate gains made during operations.
About 25 British troops have been killed in the southern region in the past six months.
The tactics over the past six months have been to push the Taleban out of the lush river valleys where the insurgents have had a stronghold.
It has resulted in close-quarters fighting for British troops.
Gen McNeill, in an interview with BBC correspondent Alastair Leithead, said this had been a successful military strategy but that he was concerned the job of holding the ground would not be done effectively by Afghan national security forces.
He said some of the ground taken may have to be taken all over again next year if the Taleban regrouped over the winter.
"I think there is some chance of that because the Afghan national security forces have not been as successful in holding as we would like them to be," he said.
"We are likely to have to do some of this work again.
"It would nice if the Afghan national security force could hold it, then there's less of a chance we'll have to do it again."
Overall, Gen McNeill said the mission was on track and he was pleased with the military progress and ongoing reconstruction projects.
But he added that improving governance was not going well and more work would have to be done.
Our correspondent explained that after British troops have pushed forward, leaving Afghan national security forces behind to hold the ground, it can be difficult to maintain security.
He said Taleban fighters launch insurgent-style attacks, such as roadside bombings, which eat away at the confidence the presence of international forces is trying to achieve.
Training the Afghan army and police to a level where they can maintain Afghanistan security is the exit strategy for foreign forces.
The Ministry of Defence said: "[To consolidate gains] is precisely why we are increasing the size of our force in Helmand, which by the end of the year will be twice the size of the force that first went in."
The spokesman also stressed that a large amount of effort was being put into training the Afghan security forces.
But Chris Parker, former chief of staff of the Desert Rats, said: "The problem remains that we do not have enough resources to occupy an area that is not too far off the size of England with forces which number around 8,000 now on the British side."
He said there had been 12,000 troops stationed in Northern Ireland "when it was relatively peaceful".
Mr Parker, who has left the Army, said another key difficulty was the time it took to train the Afghan army and police force.
"I think that there are 10 British policemen helping in Helmand province at the moment and it comes back to resources," he told BBC News 24.
"Until they are ready to take over in full, we must have enough forces ready to help them on the ground. We are not there yet."