The Prime Minister warned the rebels they should not expect an easy victory when foreign forces leave in 2014, after a decade of fighting and the deaths of 422 British soldiers.
Mr Cameron was speaking in Kabul ahead of a historic three-way meeting with the leaders of Afghanistan and Pakistan to encourage them to fight "Talibanisation" in both nations.
He said the war on terror had already made progress as the proportion of threats to Britain from Pakistan and Afghanistan has fallen from 75pc to 50pc since 2006.
However, the Prime Minister said there were still huge challenges to stop the region being a "haven for international terror".
"The clear message is to the Taliban that you can't just wait this out until foreign forces leave in 2014," Mr Cameron said. "We will be firm friends and supporters to Afghanistan long beyond that."
