Lieutenant-General Nick Carter, deputy chief of Nato’s International Security Assistance Force (Isaf), said handing over control of security and the fight against the Taleban after 12 years was progressing well.
But he said that cutting the number of boots on the ground below that planned as part of the withdrawal from the country, during the summer fighting season, would be “unforgivable”.
He also said British troops could be involved in combat operations right up to the planned end of a Nato presence in the country at the end of next year.
He spoke as forces of president Hamid Karzai, carrying out their first engagement without western help, reportedly had to call in an Isaf air strike to gain the upper hand in a battle against drug smugglers which left more than 100 dead.
Lt-Gen Carter said: “It would be unforgivable if we allowed the gains of the last three years to be lost.
“Our judgment is we have to manage this in a way that retains confidence. Precipitating withdrawal that is not in line with the current plan will damage Afghan confidence.”
The British presence in the country will be almost halved by the end of this year to 5,200. All combat operations in the country should be over by the end of 2014, leaving Afghan forces in control.
Last week, Lance-Corporal Jamie Webb, 24, from 1st Battalion the Mercian Regiment, became the 441st British serviceman to be killed since operations in Afghanistan began in October 2001.
Brigadier Bob Bruce, commander of Task Force Helmand, told reporters it was the right time for the Afghans to take over security, albeit with the option of British back-up.
“We know for a fact there is no military solution to the insurgency; there is no way the military is going to win a counter-insurgency [war] because it is essentially a political issue,” he said.
“It is a battle of offers: the offer the government makes to the people and the offer the insurgents make to the people.”