The UN-backed tribunal set up to try the killers of former Lebanese premier Rafiq Hariri announced Tuesday it was adding a fifth suspect to the trial of four Hezbollah members accused of his murder.

The four went on trial in absentia before the Special Tribunal for Lebanon last month for the massive 2005 car bombing that killed billionaire Hariri and 22 others, including a suicide bomber.

Although the attack was initially blamed on pro-Syrian Lebanese generals, the court in 2011 issued arrest warrants against Mustafa Badreddine, 52, Salim Ayyash, 50, Hussein Oneissi, 39, and Assad Sabra, 37, all members of the Syrian- and Iranian-backed Shiite movement Hezbollah.

A fifth suspect, Hassan Habib Merhi, 48, also an alleged Hezbollah member, was indicted last year.

Merhi is charged with coordinating the preparation and delivery of a video claiming false responsibility for the attack that was sent to pan-Arab broadcaster Al-Jazeera with a view to protecting the real killers.

His case will now be added to that of the four others, STL judge David Re said on Tuesday.

"The interests of justice must favour joining the two cases," Re said, according to a statement from the court based just outside The Hague.

"The trial chamber has therefore decided that the two cases should be joined and tried on the same indictment," the judge said.

Hariri, Lebanon's Sunni prime minister until his resignation in October 2004, was on his way home for lunch when a suicide bomber detonated a van full of explosives equivalent to 2.5 tonnes of TNT as his armoured convoy passed.

The February 14, 2005 seafront blast led to the establishment by the UN Security Council of the STL in 2007.

So far, none of the suspects have been arrested. Hezbollah's leader Hassan Nasrallah has dismissed the tribunal as a US-Israeli plot, and vowed that none of the men will ever be caught.

The STL is the world's only tribunal able to try suspects in absentia.