The UK Defence Secretary admitted today that the scale of the task in pacifying southern Afghanistan had been underestimated by Britain and Nato.

Des Browne said that the Government had been under no illusions that the operation would be tough, but it had proved even tougher than predicted.

"We always knew the south (of Afghanistan) would be hard. We do have to accept that it’s been even harder than we expected," he said, in a speech to the Royal United Services Institute in London.

"The Taliban’s tenacity in the face of massive losses has been a surprise, absorbing more of our effort than we predicted it would and consequently slowing progress on reconstruction."

Mr Browne praised the progress already made in the north and east of Afghanistan in improving security, building schools and hospitals, and extending the writ of the Afghan government. But he said there had been "very little progress in governance and reconstruction" in the south of the country, although the lack of progress there was perhaps "inevitable at this stage".

Mr Browne also acknowledged Nato's difficulty in raising extra troops to make a success in the south. He challenged Nato nations to "reaffirm" their commitment to seeing the task through.

"Nato have estimated they need 2,500 more troops, 1,000 of whom are combat troops. We are still in the middle of the process of finding them.

"There is no denying it has been difficult and that we are not there yet."

Mr Browne said that there were understandable concerns among member countries at bolstering the reconstruction and stability force in Afghanistan because of the risks posed to troops in the country.

"But those of us who are already fully committed in the south - ourselves, the Canadians, the Dutch and the Estonians and the ever-present Americans - must remind our partners that it was their agreement of support that brought us to this point," he told the audience.

"The mission is vital, as it has always been. In fact, it is vital not only for Afghanistan but also for the threat that a lawless Afghanistan poses to the region and to the world and also, now that NATO has taken it on, for NATO’s own credibility."

Mr Browne said the mission had reached the point where they were tackling the most difficult region, the lawless south.

"NATO nations must decide whether to back their investment, reaffirm their original intent and to send a clear signal to the Taliban and to the Afghan population that NATO has an alliance as strong and determined to see the task through," he added.

In a guarded reproach to those countries who were lagging over offering troops, Mr Browne said that the fundamental point was that NATO was an alliance, and that when it decided to use military force, "all partners should be prepared to face equal risk".

He added: "We must persuade our partners and help them to persuade their publics" about the task in hand and help push the message to the Afghan people that NATO was there to effect"real change" across the whole of society.

He stressed that Nato’s foes had been "fought to a standstill. They cannot beat us".

He also rejected the idea that Nato forces were bound to fail in the same way as the Russians and British previously had in Afghanistan, saying that the objective of the Nato force was different - it was trying to convince Afghan villagers, some of whom were being paid with drugs money to fight against Nato troops, that their future lay with the democratically elected government and the rule of law, rather than with the tyranny of the Taleban or the exploitation of the drug barons.

He praised the 8,000 British troops in the south as superb professionals, saying they were probably the best equipped in the world to achieve such a difficult balancing act. "Success won’t be what we understand to be security and prosperity and proper governance, but it will be progress and it will be massively worth it," he added.

The widely-trailed speech came hours after three suicide attacks mounted by Taleban fighters killed dozens of Afghan civilians and four Canadian soldiers who had been handing out pens and sweets to children.

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