Senator John Kerry is to visit Pakistan late on Tuesday on a mission to resolve a diplomatic row over a US official accused of murdering two Pakistanis.
The chairman of the US Senate Foreign Relations Committee helped spearhead a record $7.5 billion aid package for Pakistan and is respected in the nuclear-armed country on the front line of the US-led war on Al-Qaeda.
He will meet government officials to reassure them of long-term US commitment to Pakistan, said a spokeswoman for the US embassy in Islamabad.
"He's coming tonight to speak with government officials about the relationship and reaffirm support for the strategic relationship," said spokeswoman Courtney Beale.
Pakistan's fragile ties with the US have gone into crisis since police arrested Raymond Davis, who confessed to shooting dead two men in self-defence on a busy street in the eastern city of Lahore on January 27.
Washington says he is a diplomat who should be released immediately in keeping with international law.
The United States has postponed a round of high-level talks with Afghanistan and Pakistan following failed attempts to get Davis out and US lawmakers have threatened to cut payments to Pakistan unless he is freed.
Kerry's visit aims to look for a resolution to the issue and to tone down the pressure on Pakistan's weak coalition government, one US official said on condition of anonymity.
Pakistan's government is under enormous pressure to put Davis on trial in a country awash with anti-American sentiment.
A court on last Friday remanded him in custody until February 25 and police accused him of cold-blooded murder.
Hundreds of Pakistanis have taken to the streets demanding that Davis be hanged over the killings. A third Pakistani man was run over and killed by a US consulate vehicle that made a failed attempt to recover Davis.
On Thursday, the Lahore high court is expected to reconvene after giving the foreign ministry 15 days to answer on whether he has diplomatic immunity.