LONDON: Scotland's first minister has defended his government's decision to free the Lockerbie bomber after a blistering attack from the FBI chief and insisted that US-Scottish ties would remain strong.

Robert Mueller, the director of the US Federal Bureau of Investigation, said in a strongly worded letter to the Scottish justice minister that releasing Abdelbaset Ali Mohmet al-Megrahi "gave comfort to terrorists around the world".

Mueller said the decision to free Megrahi, who has terminal cancer, on compassionate grounds made a "mockery of justice".

But Scotland's First Minister Alex Salmond said he "clearly disagreed" with the FBI director.

"It is difficult for people sometimes in the United States to recognise that it is a different legal system, but it is a different legal system, it is a Scottish legal system and therefore we have to follow the tenets of Scottish justice," Salmond told Sky News.

He also dismissed a website calling for a boycott of his nation and its products over the Megrahi case.

"Many, many things appear in the blogosphere, but what we're talking about is the real world and in the real world the relationship between Scotland and the United States is strong and enduring," he said.

A government spokesman said the decision to release Megrahi had been reached on the basis of Scotland's "due process, clear evidence, and the recommendations from the parole board and prison governor".

"Compassionate release is not part of the US justice system but it is part of Scotland's," the spokesman said.

The Lockerbie case strikes a personal chord for Mueller, a former prosecutor who played a major role in investigating the 1988 bombing of Pan Am Flight 103 over the Scottish town. The majority of the 270 victims were American.

Criticism of the decision also came from Scotland's former first minister Jack McConnell, who said it was a "grave error of judgement".

"The way in which the decision has been made and the decision itself has damaged the reputation of the Scottish justice system," McConnell told the BBC.

"It's damaged that reputation, but much more significantly it's also damaged the reputation of Scotland internationally."

McConnell is a member of the Labour party, whose administration preceded the current one formed by the pro-independence Scottish National Party.

The semi-autonomous Scottish government, based in Edinburgh, can take decisions independently of London on justice matters but not foreign affairs.

In its reply to Mueller's criticism, the Scottish government said that before taking the decision, Scottish Justice Secretary Kenny MacAskill "could not have consulted more widely -- he spoke with the US families, the US attorney general, Secretary of State (Hillary) Clinton and many others".

The son of Libyan leader Moamer Kadhafi, Seif al-Islam, claimed following the release that the issue of Megrahi "was always on the negotiating table" in talks with Britain over Libya's huge reserves of oil and gas.

But Seif al-Islam's Kadhafi Foundation, which financed Megrahi's legal defence, played down the comments.

Its director Yussef Sawan said on Saturday the decision to free Megrahi was taken "solely on humanitarian grounds".

Britain's Business Secretary Peter Mandelson -- Prime Minister Gordon Brown's de facto deputy -- said suggestions that a deal had been struck on Megrahi were "offensive".

The British government is under pressure to explain its role in Megrahi's release, but it says the decision was a matter solely for Scotland.

Employment Minister Jim Knight said he hoped there was no "fall-out" over the release for Scotland and Britain "in terms of our relationship with the US which is a key relationship for us".

Britain and the United States have condemned the joyous scenes when Megrahi returned to Tripoli on Thursday. Megrahi has also been shown on Libyan TV meeting and embracing Kadhafi.