The threat posed by the Taleban in Afghanistan has been under-estimated, the UK defence secretary has admitted.
Des Browne said the fight had been "even harder than we expected" but insisted Nato was pursuing a "noble cause" and its mission would succeed.
Addressing the Royal United Services Institute, he urged other Nato members to respond to a call for 2,500 extra troops for Afghanistan.
The Nato force in the southern Helmand province includes some 4,000 UK troops.
"Nato nations must decide whether to back their investment, re-affirm their original intent and send a clear signal that Nato as an alliance is strong and determined to see the task through," Mr Browne said.
Poland has already announced it will send 1,000 extra soldiers to Afghanistan next February, joining 100 already on the ground.
Meanwhile, Canada has promised its contribution to the Nato force will remain in Afghanistan.
Foreign Minister Peter MacKay made the pledge after four Canadian soldiers were killed by a suicide bomber on Monday. Its 2,500-strong contingent is to be reinforced by an extra 200 soldiers and tanks.
Dutch, Danish, Estonian and US troops are also currently in the south.
Mr Browne said critics who cited the failure of the Soviet Union in Afghanistan in the 1980s and the British in the 1800s did not understand the nature of the mission.
"We are not invading," Mr Browne said. "We are there at the invitation of a government which has legitimacy and support."
Mr Browne said security in the north and west of the country had brought "real change" with new schools and hospitals, more jobs and the return of refugees.
But he acknowledged "relatively little progress" had been made in the south, the traditional Taleban heartland and the area where most of Afghanistan's opium production is concentrated.
About 4,000 British soldiers are based in southern Afghanistan
"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 said.
But he added: "The Taleban'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."
UK troops have been involved in heavy fighting with the Taleban after taking over from a US-led coalition in southern Afghanistan in July.
This month 19 servicemen have lost their lives, including 14 who died when an RAF Nimrod reconnaissance aircraft crashed.
Mr Browne said the Taleban were adept at forming "alliances of convenience" with drug barons and criminal gangs who recruited foot soldiers from among Afghan tribesmen.
"If we cannot persuade them to put down their guns, then we will struggle to make progress, and there will be a real danger that their deaths will motivate others to join the fight, and potentially turn this into a conflict of a different kind," Mr Browne said.
"But I do not believe we are at that stage."
He said Nato's progress rested on convincing ordinary people to "back peace, to back the view of the future represented by the Afghan government".
After the speech, Mr Browne called on neighbouring Pakistan to do more to prevent insurgents crossing into Afghanistan.
Later, shadow defence secretary Liam Fox told the BBC a "major diplomatic effort" involving the prime minister and foreign secretary was needed to put pressure on the UK's Nato allies to supply troops.
Liberal Democrat foreign affairs spokesman Michael Moore suggested more British soldiers may have to be sent if Nato's request went unheeded.
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