General James Bucknall said the coalition had to hold on to recent gains, and should not send “conflicting signals” about its commitment.
The intervention comes amid debate over when the military surge implemented by US President Barack Obama should be wound down.
David Cameron announced last month that about 450 UK personnel are to be pulled out by the end of the year, and there have been hints that further cuts could follow.
The elimination of Osama bin Laden has added impetus to calls for operations to be scaled back.
In an interview, Gen Bucknall – deputy to US General David Petraeus in Afghanistan – said the 140,000 Nato troops now deployed could be cut this year by trimming roles such as cooks and engineers.
But “in broadest terms”, the reinforcements which arrived last year had to stay, he said.
“What we are doing is reaping the benefits of having the resources in place to match the strategy we have always had. Many of those resources only hit the ground in autumn 2010.
“We need, in broadest terms, that set of resources in place for two winters and two fighting seasons, which would mean we are talking about autumn 2012. This is not the time to send conflicting signals on commitment to the campaign.”
There has been a spike in violence over the past fortnight as the Taliban has begun its usual spring offensive following the end of the opium poppy harvest.
Gen Bucknall said months of nightly special forces raids had weakened the insurgency by capturing or killing leaders and seizing caches of arms and explosives.