The Afghan government would quickly be overthrown if Nato troops pulled out of the country now, British foreign secretary David Miliband said on Friday.
"If international forces leave, you can choose a time — five minutes, 24 hours or seven days — but the insurgent forces will overrun those forces that are prepared to put up resistance, and we would be back to square one," he told the Guardian newspaper.
At the end of a visit to Kabul for the inauguration of president Hamid Karzai, Miliband said Afghans were "sad that they need anyone, but they are passionate that my goodness they do, because if we weren't here, their country would be rolled over."
More than 230 British troops have been killed in Afghanistan since 2001, and British public opposition has grown to involvement in the international coalition fighting the Taliban.
"What we have to do is explain to people that the costs of staying are real, but they are less than the costs of leaving," Miliband said.
Prime minister Gordon Brown has insisted British troops are in Afghanistan to protect Britain from terrorism. He said earlier this month that Britain and its allies must not walk away, but expand training of Afghan security forces so they can eventually take over responsibilities from foreign forces.
Miliband said western involvement in Afghanistan would need to continue until the transition to Afghan control was complete.
"My argument is we stay for a purpose, for a period, for progress," he said. "The goal is hard, but the goal is clear."
