A letter from senators including Edward Kennedy and John Kerry expressed concern that he could soon be freed.

Kenny MacAskill will decide in the next two weeks whether he can be released or transferred to Libyan custody.

Megrahi, who has terminal prostate cancer, is expected to formally abandon an appeal against his conviction.

The justice secretary is considering an application to release 57-year-old Megrahi on compassionate grounds, as well as a Libyan government request for a transfer to allow him to serve out his sentence in his homeland.

The move to drop Megrahi's appeal, expected at an Edinburgh court hearing later, would remove a possible obstacle to the prisoner transfer agreement.

A prisoner transfer cannot take place while legal proceedings remain active in the Scottish courts.

Last year Megrahi's lawyers asked the Appeal Court in Edinburgh to release him on bail on compassionate grounds, but the application was rejected by the three judges.

Mr MacAskil is said to have cleared his desk to focus on making the decision on the fresh applications.

Reporter say there has been outrage in the US at the possibility of Megrahi's release.

Of the 270 people killed in the 1988 bombing of Pan Am flight 103, 189 were US citizens.

The senators' letter calls it "a horrific act of international terrorism", adding that "until the tragic events of September 11, 2001, no terrorist act had killed more American civilians".

It says: "We know that the Scottish government shares our commitment - and the world's - to support justice and oppose acts of terrorism.

"That is why we urge you to ensure that [Abdelbaset Ali al-Megrahi] serves the remainder of his sentence in prison in Scotland."

US Secretary of State Hillary Clinton previously pressed Mr MacAskill to keep Megrahi behind bars.

Scottish Labour and Liberal Democrat calls for parliament to be recalled for a debate on the issue have been rejected.

Holyrood presiding officer Alex Fergusson said: "I have weighed up all the factors very carefully and have taken the decision not to recall parliament at this time."

First Minister Alex Salmond said those urging the recall of parliament had to realise that Mr MacAskill would not be influenced by politics and once he had made his decision he would announce it to parliament in the normal way.

The first minister said: "It is a matter in which the cabinet secretary for justice operates in a judicial way. He is not influenced by world power politics, by press speculation or party politics."

Mr Salmond added that Mr MacAskill had a "weighty responsibility".

The Foreign Office has said it sees no international legal obstacle to prevent the Scottish Government returning Megrahi to Libya.

Megrahi is serving a life sentence of at least 27 years after being convicted in 2001.