David Cameron has warned the Taliban not to "wait it out" in Afghanistan until troops withdraw in 2014.
The PM said British support for the country would remain long after the planned exit date, which will see all international combat forces leave.
Afghan security forces are "capable and large" but Britain's support for the nation will continue for many years, he said.
"The Taliban should be in no doubt that we will handing over to a very large, very capable Afghan security force," Mr Cameron said at a joint press conference with Afghan president Hamid Karzai, held in the gardens of the presidential palace.
He added: "You cannot wait this out until foreign forces leave in 2014 because we will be firm friends and supporters of Afghanistan long after 2014."