The US and UK have warned Libya over the welcome given to the Lockerbie bomber after his release from prison on compassionate grounds, BBC reported.
Both countries said they were concerned at the sight of Abdelbaset Ali al-Megrahi being greeted by a jubilant crowd at Tripoli airport.
They said they had warned Libya about the sensitivity of the issue.
Libyan leader Muammar Gaddafi has yet to comment on Megrahi, who has not been seen in public since returning.
He is believed to have been resting at his family home after his journey from Scotland.
Families of American victims of the bombing in 1988, when Pan Am Flight 103 was blown up over the Scottish town of Lockerbie with the loss of 270 lives, are due to discuss how to protest over Megrahi's release and Libya's attitude.
They are said to be considering a demonstration against Col Gaddafi when he visits New York next month.
A British royal visit to Libya, also scheduled for September, is being reconsidered in the light of the release row.
"It is disturbing to see images suggesting that Megrahi was accorded a hero's welcome instead of being treated as a convicted murderer," said White House spokesman Bill Burton.
He said that the US had communicated to the Libyan government that "such a welcome sends the wrong message and is deeply offensive to the families of the hundreds of people who lost their lives in the Lockerbie bombing".
It has emerged that UK Prime Minister Gordon Brown wrote to Col Gaddafi on Thursday, asking for Libya to "act with sensitivity" in handling Megrahi's return.
He also made clear that the decision to release the terminally ill bomber on compassionate grounds was the Scottish government's, a Downing Street spokeswoman said.
Speaking to the BBC, UK Foreign Secretary David Miliband said: "Obviously the sight of a mass murderer getting a hero's welcome in Tripoli is deeply upsetting, deeply distressing."
Libya's response would be closely scrutinised, he added.
"It's very important that Libya knows that how the Libyan government handles itself in the next few days will be very significant in the way the world views Libya's re-entry into the civilised community of nations," Mr Miliband said.
Despite the scenes at the airport, Friday saw no sign of triumphalism from Libyan ministers, the BBC's Christian Fraser reports from Tripoli.
Most of Libya's newspapers are not published on Friday. However, the state-owned Al-Jamahiriyah offered an editorial condemning the conduct of Megrahi's trial and describing him as a "political hostage".
Hundreds of people turned out to meet Megrahi's plane as it landed in Tripoli, many waving flags.
Megrahi was met by Col Gaddafi's son, Seif al-Islam Gaddafi, who thanked both the Scottish and British governments for their "brave stance" although the British Government has insisted the decision to release Megrahi was a purely Scottish affair.
The younger Gaddafi added that there was a "considerable amount of new evidence" to show Megrahi was innocent.
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