Gordon Brown is expected to urge Pakistan to take more action against militants sheltering in the country when he meets its prime minister later.
UK Prime Minister Gordon Brown has pledged more support for Pakistan's counter terror efforts after talks with Prime Minister Yousef Raza Gilani.
In talks at Downing Street, he is also set to press Yousef Raza Gilani to take more of a role in the Afghan campaign.
At a joint press conference, Mr Brown told Mr Gilani: "This is your fight but it is also Britain's fight."
Last weekend, Mr Brown called for a greater effort against the presence of al-Qaeda in Pakistan's border area.
Mr Brown did not repeat his call at the weekend for Pakistan to do more to track down Osama Bin Laden.
Mr Gilani is expected to say that his security forces are doing what they can to tackle extremism but need more help.
Mr Gilani said no useable intelligence had been provided, adding: "I don't think Osama Bin Laden is in Pakistan."
He has already expressed concern that the extra Nato troops being sent to Afghanistan could push more militants over the border into Pakistan.
The Pakistan PM also said he wanted "more clarity" from the Americans on US President Barack Obama's new Afghanistan war strategy before his country could take action on it.
BBC world affairs correspondent Caroline Hawley said the visit comes at a crucial time, as the West responds to the interconnected problems of Afghanistan and Pakistan.
Mr Brown began the Downing Street press conference by praising Pakistan's efforts to counter the Taliban and acknowledged the "huge sacrifices" made by the country in fighting extremism.
The Taliban's use of sanctuaries in Pakistan to attack Nato forces in Afghanistan is a major concern for officials, our correspondent said.
He said aid being provided by Britain would go into reconstruction, education and the relocation of people displaced by fighting in Pakistan's turbulent border regions with Afghanistan.
Troop surge
He then pledged £50m to help Pakistan achieve the "long-term stabilisation" of the border region.
Gordon Brown said last weekend that questions must be asked about why nobody has been able to "spot or detain" either al-Qaeda leader Osama bin Laden or his deputy Ayman al-Zawahiri in the eight years since the September 11 attacks in the US.
They are thought to be hiding in the border area.
Announcing the UK's decision to send 500 more service personnel to Afghanistan, Mr Brown again highlighted the need for more action from Pakistan.
He welcomed its push into the South Waziristan region but said it needed to "step up" efforts against the Pakistan Taliban and al-Qaeda.
US President Barack Obama also pledged to work with Pakistan when he committed an additional 30,000 soldiers to the mission in Afghanistan.
