Britain is to boost its presence in Afghanistan by 300 troops, Gordon Brown announced last night, as he called for other countries to increase their sharing of the military burden.

In a Commons statement, the Prime Minister revealed that the UK military presence, following advice from commanders on the ground, would rise from just over 8000 to 8300 in the run-up to Afghan elections.

In making the announcement, he made it clear allies had to increase their troop numbers.

"Forty-one countries are involved in the Nato mission but the burden is not always shared equally. It is vital that all members of the coalition contribute fairly," Mr Brown told MPs. The issue is to be considered at a Nato summit next April.

His statement came after he visited Afghanistan over the weekend, holding talks with Hamid Karzai, the Afghan President, and meeting British troops.

It was then that the PM signalled that a reserve British contingent of around 300 troops would be put on a more formal footing. The Ministry of Defence said last night the deployment would include two companies from the 2nd Battalion The Royal Regiment of Fusiliers, stationed in Cyprus.

The Prime Minister insisted that, despite recent UK casualties, there had been "progress" in the country.

Mr Brown told MPs: "To reinforce this progress, and having been briefed on the decision by the British commander - as is his right - to call forward reserves to work with our allies and deploy them on a temporary basis in the campaign in central Helmand, the Defence Secretary and I have decided on advice from the defence chief to approve until August - including the period of preparation for the Afghan elections - an increase in the number of British troops deployed to Afghanistan from just over 8000 to 8300."

He explained how there was a "chain of terror" that linked the mountains of Afghanistan and Pakistan, which must be "broken".

The commitment to counter terrorism must be just as strong in Pakistan as in Afghanistan, he insisted.

"The time has come for more action, not more words. We will offer our support for Pakistan but that government must act against the terror networks based on its soil."

Mr Brown added: "Pakistan's own future depends on action against those within its borders who are bent on the destruction of its elected government."

In response, David Cameron said there were "real causes for concern".

The Conservative leader pressed Mr Brown to outline the UK's contribution to General David Petraeus's review of American involvement in the country, adding: "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 said Britain's armed forces were doing a "great job" but called on them to be given a "realistic mission".

"Shouldn't it focus predominantly on security and rooting out terrorist training, not an unrealistic objective of completely transforming a society thousands of miles away?" Mr Cameron asked.

Nick Clegg for the Liberal Democrats welcomed the Prime Minister's commitment to Afghanistan, India and Pakistan and offered his support for the temporary increase in troops.

However, he made clear that "any lasting peace" in Afghanistan needed a regional agreement.

"We need to start talking now to China, Russia, and also to Iran," he added.

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