A UN-backed tribunal will on Friday sentence a Hezbollah member convicted of the 2005 assassination of former Lebanese premier Rafic Hariri, with prosecutors demanding a life term.

Salim Ayyash, 57, was found guilty in absentia of murder on August 18 by the Netherlands-based Special Tribunal for Lebanon over the suicide bombing that killed the Sunni billionaire politician and 21 other people.

Ayyash remains on the run, with Hassan Nasrallah, the head of the Shiite Hezbollah movement, refusing to hand him over, alongside three other defendants who were eventually acquitted.

Prosecutors in a hearing in November said a life term was the "only just and proportionate sentence" for Ayyash, given that it was the "most serious terrorist attack that has occurred on Lebanese soil."

They have also demanded the seizure of Ayyash's assets.

Hariri served as Lebanon's prime minister until he resigned in October 2004.

He was killed in February 2005 when a suicide bomber detonated a van filled with explosives as his armoured convoy drove past. As well as those killed, another 226 were wounded in the blast.

In their long-awaited ruling in August, judges said there was sufficient evidence to show that Ayyash was at the centre of a network of mobile phone users who scoped out Hariri's movements for months before his assassination.

- 'Culture of impunity' -

But there was not enough evidence to convict Ayyash's co-defendants Assad Sabra, Hussein Oneissi and Hassan Habib Merhi, they said.

The judges added that there was no proof to tie Hezbollah's leadership or its allies in Damascus to the attack.

Legal experts said the sentencing was still important, even without Ayyash in the dock.

"In absentia trials are of course not the ideal way of dispensing international justice," Christophe Paulussen, senior researcher at the Asser Institute in The Hague, told AFP.

International tribunals were like "a giant without arms and legs" since they relied on states to arrest suspects and could not enforce orders themselves.

"But even with this handicap, the STL has now at least established a very authoritative judicial record about what happened 15 years ago, thus assisting the Lebanese society in moving away from a culture of impunity towards one of accountability," said Paulussen.

The UN Security Council agreed in 2007 to establish the court, billed as the world's first international tribunal set up to probe terrorist crimes.

It opened its doors in 2009, although the Hariri trial itself did not formally start until 2014.

The court has cost at least $600 million to operate and has so far heard only four cases, two of them for contempt of court about news reports with information about confidential witnesses.

Ayyash faces a separate case at the tribunal over three other deadly attacks on Lebanese politicians in 2004 and 2005.