US Defence Secretary Robert Gates is in Pakistan for talks on how to combat the growing threat from militants.
Mr Gates is making his first visit to Pakistan, a key US ally, since President Obama took office last year.
The trip comes at a crucial time in the fight against al-Qaeda and the Taliban, with the US planning to send tens of thousands more troops to Afghanistan.
Talks are expected to focus on how to combat militants in Pakistan's tribal areas bordering Afghanistan.
Correspondents say the United States would like to see Pakistan expand an offensive against Taliban fighters in the area.
Mr Gates is also expected to raise concerns that Pakistan is delaying the extension of visas of US officials.
The BBC's Syed Shoaib Hasan in Islamabad says the main issue on Pakistan's agenda is likely to be the increased use of drone strikes by the US in Pakistan's tribal areas.
Hundreds of people have died in the attacks, which have stoked deep resentment of the US among many Pakistanis.
The US defence secretary flew into Islamabad on Thursday from talks in Delhi.
Arriving in the Pakistani capital, Mr Gates said he would reassure his hosts that the United States is "in this for the long haul".
His visit comes amidst a slight deterioration in relations between the two allies. In an article published in a Pakistani newspaper on Thursday, Mr Gates refers to a "trust deficit".
He is expected to hold discussions with Prime Minister Yusuf Raza Gillani and President Asif Zardari.
Our correspondent says there is public concern in Pakistan over the expanding presence of US government personnel in the country.
Islamabad is expected to seek assurances that these personnel will not engage in activities beyond their designated duties.
At the same time our correspondent says the government may also ask for quicker delivery of aid packages, which have been held up recently and are greatly needed to boost the economy.
Mr Gates, meanwhile, will argue that drone strikes are the only effective measure against the Taliban and that stationing US personnel in Pakistan is essential to ensure it receives the expertise needed to implement the massive US aid package.
He is also expected to tell Pakistan that it could do more against top Taliban leaders - some of whom are alleged to have close links to Pakistan's ISI intelligence service.
"You can't ignore one part of this cancer and pretend that it won't have some impact closer to home," Mr Gates told reporters travelling with him from India.
Pakistan has been an important US ally in the fight against militants in South Asia since the 11 September 2001 attacks in the US.
Mr Gates was forthright in his warnings about the al-Qaeda threat
On Wednesday, Mr Gates said al-Qaeda was trying to destabilise the whole of South Asia hoping to provoke war between India and Pakistan or to provoke instability in Pakistan.
In comments which might unsettle some in Pakistan, Mr Gates suggested India might not show restraint if it suffered another attack like the one in 2008 on Mumbai.
Blamed on Pakistan-based militants, the attack killed more than 160 people.
Peace talks between the two countries, who have fought three wars since independence from Britain, have been on hold ever since.
Mr Gates described militant groups in South Asia - the Taliban in Afghanistan and in Pakistan, and the Pakistan-based group Lashkar-e-Taiba - as "a syndicate of terrorist operators intended to destabilise this entire region".