Gordon Brown pledged to defeat the Taliban on the battlefield and in people's hearts and minds in his opening remarks to the London Afghanistan conference.
The prime minister directly addressed the leaders of the Taliban, whose leadership of the insurgency in Afghanistan has thwarted coalition forces since the country was invaded in October 2001.
"Today our message to al-Qaida must be clear," Mr Brown said.
"It is the same message we sent to all those who pursue violent and extremist ideologies that pervert the true Islamic faith.
"We will defeat you. And we will defeat you not just on the battlefields but in the hearts and minds of the peoples of this world and particularly people in Afghanistan."
The prime minister's comments came as the London conference on Afghanistan, to which 70 countries were invited, got underway.
His uncompromising attitude comes despite a key purpose of today's conference - securing international agreement to a new approach towards non-ideological fighters, in a bid to woo these men away from the Taliban.
The conference was also expected to agree debt cancellation and push Afghan president Hamid Karzai to agree to implementing anti-corruption measures.
United Nations secretary-general Ban Ki-moon pressured the Afghan government to make progress on a range of issues, including aid, economic development, reintegration and the expansion of security forces.
He told the conference: "The Afghan government and people must take the lead on translating commitments into reality."
Mr Karzai responded by acknowledging the debt the Afghan people had for its international partners.
But he insisted that "Afghan leadership" and "Afghan ownership" were the priority, as all involved wanted to see his government take "solid strides towards ownership of [Afghanistan's] security and development".
"We must reach out to all our countrymen - especially our disenchanted brothers who are not part of al-Qaida or any terrorist networks," he added.
A national council of reconciliation and reintegration will be established to this end, to be followed by a peace jirga. King Abdullah of Saudi Arabia is being asked to play a "prominent role" in this process, Mr Karzai said.
His remarks contrasted with Mr Brown's confrontational approach to the uncompromising Taliban leadership.
"We will defeat you in any and every country where you seek refuge," the prime minister said.
"For today this conference shows the people of the world speak as one. United and resolute we will defeat terrorism.
"United in supporting the government of Afghanistan to deliver peace and security for its people. And united in our determination and resolve to do what is right to support all those determined to build a more secure and prosperous life free of terrorism."