David Cameron has declined to meet a group of US senators to discuss concerns about the release of the Lockerbie bomber.
Four senators from New York and New Jersey are calling for an investigation into oil giant BP's role in lobbying for the release of terminally ill Abdul Baset Ali al-Megrahi in order to smooth the way for a massive exploration deal.
A Downing Street source said Mr Cameron was unable to meet the group because of the tight schedule for his two-day US visit, taking in talks with President Barack Obama at the White House and United Nations Secretary General Ban Ki Moon in New York.
The source said that Mr Cameron "understands" the senators' concerns. Before leaving for the US on Monday, he said that the decision to release cancer sufferer Megrahi on compassionate grounds last year had been "completely and utterly wrong".
The White House said that Mr Obama's talks with the PM were likely to touch on the issue of the Lockerbie bomber's release and whether BP played any part in it.
BP has confirmed it spoke to the previous government about the "negative impact on UK commercial interests" caused by slow progress on a prisoner transfer agreement. But it denies any involvement in the Scottish Executive's decision to release al-Megrahi.
Mr Cameron has pledged his administration will "engage constructively" with a planned Senate foreign affairs committee hearing later this month on the Megrahi release.
The majority of the 270 people killed in the bombing of Pan Am flight 103 over the Scottish town of Lockerbie in 1988 were American.
Megrahi is the only person ever convicted of involvement in the atrocity and his return to a hero's welcome in Tripoli triggered fury in the US, which has only been heightened by his continued survival.