|Al-Megrahi, who was released from a Scottish prison in 2009, is reported to be near death at his home in Tripoli
Libyan opposition leaders must deal with the case of Abdel Baset al-Megrahi, the only person convicted over the 1988 Lockerbie bombing, Hillary Clinton, the US secretary of state, has said.
In a meeting with senior members of Libya's National Transitional Council in Paris on Thursday, Clinton said the US would be watching closely how the case was handled.
Clinton's comments came amid growing calls from US politicians and leading Republican presidential candidates for al-Megrahi's return to prison or even extradition.
Al-Megrahi, who is suffering from prostate cancer, was released on compassionate grounds two years ago, after serving eight years of a life sentence in Scotland for the passenger jet bombing that killed 270 people, many of them US citizens.
Doctors had said he would die of cancer within three months.
"The United States categorically disagrees with the decision that was made two years ago by the Scottish executive to release al-Megrahi and return him to Libya," Clinton told reporters.
"We have never wavered from our disagreement and condemnation of that decision. He should be behind bars."
In her meetings, Clinton made clear to Libyan opposition leaders Mustafa Abdul-Jalil and Mahmoud Jibril that the US believed al-Megrahi should never have been freed.
His return to a hero's welcome in Muammar Gaddafi's Libya remains problematic, US officials said.
Clinton said the US also was seeking the Libyan interim government's assistance for more information about possible accomplices in the planning or execution of the bombing in December 1988.
Al-Megrahi, a staunch Gaddafi supporter, is now reported to be near death at his home in Tripoli, the Libyan capital.
The NTC says it will not deport him even though Chuck Schumer, a US senator, has asked Clinton to make the release of billions of dollars in frozen assets from the Gaddafi government contingent on al-Megrahi's return to jail.
The US state department said on Wednesday that Clinton would press the opposition on the case, but would not link it to the assets, given the immediate priorities such as securing and stabilising Libya.