Britain's military commander and ambassador in Afghanistan are being "defeatist" by thinking the war against the Taliban cannot be won, the US Defence Secretary, Robert Gates, said as Washington sought more troops for the conflict.

Brigadier Mark Carleton-Smith had said that the war could not be won and that the goal was to shrink the insurgency so it was no longer a strategic threat and could be dealt with by the Afghan army. Britain's ambassador, Sherard Cowper-Coles, saw an "acceptable dictator" as the best solution, with a troop surge only creating more targets for the Taliban.

Their comments were echoed by the top UN official in Kabul, Kai Eide, who said success was only possible through dialogue and political efforts.

Security, however, has deteriorated markedly in the country over the past two years. "There certainly is no reason to be defeatist or to underestimate the opportunities to be successful in the long run," Mr Gates said yesterday on his way to Europe to meet defence ministers.

He said: "Part of the solution is strengthening the Afghan security forces," adding that he would negotiate with members of the Taliban willing to work with the Kabul government. He compared that to reconciliation efforts in Iraq, where tribal leaders have switched sides to fight the insurgency.

The Taliban have repeatedly rejected the idea of talks unless all 70,000 foreign troops leave the country.

Meanwhile, the Taliban spokesman, Qari Mohammad Yousuf, denied reports that negotiations had taken place between the Taliban and the Afghan government in Saudi Arabia.