The White House says President Barack Obama has voiced his disappointment directly to British Prime Minister Gordon Brown over the release of the Lockerbie bomber.
Mr Obama made the remarks in a telephone call to Mr Brown on Thursday.
Abdelbaset Ali al-Megrahi was jailed for the 1988 attack which claimed 270 lives. He was freed on compassionate grounds last month. He has terminal prostate cancer.
He then returned to his native Libya, where he was given a hero's welcome.
The decision to release was taken by the Scottish government and the British government insists it did not intervene.
Last month, Mr Obama said the release of the bomber was "a mistake", but had not expressed this personally to Mr Brown.