US President Barack Obama has welcomed the filing of an indictment over the killing of former Lebanese Prime Minister Rafik Hariri and 22 other people in a bomb attack nearly six years ago.

He called for the United Nations-backed special tribunal for Lebanon to be allowed to continue its work.

'This action represents an important step toward ending the era of impunity for murder in Lebanon, and achieving justice for the Lebanese people,' Mr Obama said in a statement.

'I know that this is a significant and emotional time for the Lebanese people, and we join the international community in calling on all Lebanese leaders and factions to preserve calm and exercise restraint,' Mr Obama said, calling the United States a 'strong friend' of Lebanon.

The prosecutor of the tribunal, Daniel Bellemare, has issued a draft indictment but its contents have not been released.

However Lebanese officials and Western diplomats have said they expected the court to accuse members of the powerful Shia group, Hezbollah, of involvement in the assassination.

Hezbollah denies any such role and has described the tribunal as an 'Israeli tool'.

There are fears that the indictment could trigger violence after the Lebanese government collapsed last week.