Gordon Brown has told MPs that Britain is to boost the number of troops serving in Afghanistan by around 300.
The prime minister used a Commons statement on Monday to say that the forces would reinforce "progress" already made, with reserves being called forward for deployment on a "temporary basis".
Until August, including the period of preparation for elections in Afghanistan, the number of British troops will rise from just over 8,000 to around 8,300.
But following his weekend visit to the region, Brown was critical of the lack of equal "burden sharing" among Nato countries.
Brown said 41 countries were involved in Afghanistan "but the burden is not always shared equally".
It was "vital that all members of the coalition contribute fairly" and this would be looked at by Nato at a meeting next April, he added.
The prime minister said there was a "chain of terror" that linked the mountains of Afghanistan and Pakistan, which must be "broken".
"The time has come for more action not more words," he declared.
"We will offer our support for the democratic government of Pakistan. But that government must act rapidly and decisively against the terror networks based on its soil.
"Pakistan's own future depends on action against those within its borders who are bent on the destruction of its elected government and Pakistan's relations with its neighbours.
"Britain will work with both India and Pakistan to continue building counter-terrorism capacity."
Conservative leader David Cameron said there were "real causes for concern" on the ground in Afghanistan.
"Surely we should only send more troops if there is a proper political strategy to help deliver security, if there is more effective burden sharing with our Nato allies, and if there is a corresponding increase in the vital equipment - especially Chinook helicopters and armoured vehicles?" he asked.
Liberal Democrat leader Nick Clegg said any efforts to establish peace would need a "regional agreement", with Nato countries talking to China, Russia and Iran.