Nato has to "turn the tide" against the Taliban by next year, Gordon Brown insisted today as he warned of more bloodshed to come.
The Prime Minister backed efforts to reach out to insurgents who were willing to renounce violence.
The comments came as he opened a high-level international conference on Afghanistan in London this morning.
"2009 was a difficult year in Afghanistan - and there will be more tough times ahead," Mr Brown told delegates.
"All our forces have made great sacrifices, with hundreds of lives lost and thousands of casualties sustained. In the last year Britain alone, has suffered over 100 fatalities.
"But these sacrifices are not in vain - all the countries represented here recognise that this campaign is vital to our own national security, to the stability of this crucial region, and to the security of the world."
The Premier said the strength of the Afghan security forces was due to reach 300,000, and the US military surge would help increase Nato's force to around 135,000.
That was being complemented with a civilian "surge" intended to improve governance and boost economic activity.
But Mr Brown warned that the heightened effort had to deliver results quickly, stating: "By the middle of next year we have to turn the tide in the fight against the insurgency and also in our work to support the Afghan government in winning the trust of its people."
He said he believed the strategy being put in place would work "sooner than many expect" and the gradual process of handing over security control to the Afghans "will begin later this year".
Earlier, Afghan president Hamid Karzai suggested that his country would need UK support for another 15 years.
"With regard to training and equipping the Afghan security forces, five to 10 years would be sufficient," he told BBC Radio 4's Today programme.
"With regard to sustaining them... the time period extends to 10 to 15 years."
Speaking alongside Mr Brown on the conference platform, Mr Karzai said he hoped Afghan forces would control security in all areas of the country within five years.
"During the next two to three years we intend to focus on gradually assuming the responsibility for security in greater parts of our country," he said.
"This will allow our international partners to eventually move their security forces out of the parts secured by our own forces while refocusing their efforts on better civilian (infrastructure), economic development and rebuilding Afghanistan.
"We will spare no effort or sacrifice to lead security of our country within the next five years all over Afghanistan."
Mr Brown urged the international community to reiterate its commitment to the campaign, and endorsed Mr Karzai's plan to re-integrate Taliban fighters who are prepared to renounce violence.
"As an international community responding to President Karzai's leadership, we are today establishing an international trust fund to finance this peace and re-integration programme to provide an economic alternative to those who have none," he said.
But the Prime Minister had a hard-line message for al Qaida and other extremist groups who "perverted" the Islamic faith.
"We will defeat you and we will defeat you not just on the battlefield but in the hearts and minds of the people of this world, and particularly the people in Afghanistan," he said.
"We will defeat you in any and every country in which you take refuge."Reuse content