The US Secretary of State Hillary Clinton has arrived in Pakistan on a surprise visit aimed at soothing tensions between the two countries.
It's the first such high-level visit to Pakistan since the killing of al-Qaeda leader Osama Bin Laden on 2 May.
Islamabad is unhappy that it had no prior knowledge of the raid on Bin Laden's compound in Abbottabad.
It comes a day after the US announced it was withdrawing some of its troops from Pakistan, at Islamabad's request.
The Pentagon said it had received a request from the Pakistani government to reduce its presence in the country.
Relations between US and Pakistan are always complex and fragile but they are particularly volatile at the moment.
In Washington, suspicion is rife that some in Pakistan knew of Osama Bin Laden's hiding place.
During her visit, Mrs Clinton is to meet Pakistani President Asif Ali Zardari, army chief General Ashfaq Kayani and Ahmad Shuja Pasha, chief of Pakistan's powerful intelligence agency ISI.
The secretary of state is accompanied by chairman of the US Joint Chiefs of Staff Admiral Mike Mullen.
They are expected to demand more cooperation from Pakistan in the fight against al-Qaeda and Taliban militants.
Some in Washington believe that Pakistani intelligence works closely with violent extremist groups.