The UN-backed tribunal set up to try the killers of former Lebanese premier Rafiq Hariri has postponed the trial at least until May so a fifth suspect can be added, it said Wednesday.

Special Tribunal for Lebanon(STL) judges ordered earlier this month that Hassan Habib Merhi, 48, was to join the repeatedly delayed trial in absentia of four other Hezbollah members for the 2005 attack that killed Hariri and 22 others, including the suicide bomber.

"The judges ordered the adjournment of the trial... until at least early to mid-May to allow defence counsel for Merhi adequate time to prepare for trial and to conduct their own investigations," the STL said from its headquarters just outside The Hague.

The four initial suspects finally went on trial in January for the massive downtown Beirut bombing on February 14, 2005, that also wounded 226 others.

Prosecutors are aiming to prove the accused mens' involvement through tracking their alleged use of mobile phones before, during and after the attack.

Hariri, Lebanon's Sunni Muslim 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 backlash against his killing led Syria to withdraw its troops from the country, ending a presence that lasted nearly 30 years.

The blast also led to the establishment by the UN Security Council of the STL in 2007.

The four original suspects are Mustafa Badreddine, 52, Salim Ayyash, 50, Hussein Oneissi, 40, and Assad Sabra, 37. None of the five suspects has been arrested.

The Syria- and Iran-backed Hezbollah's leader Hassan Nasrallah has in the past dismissed the tribunal as a US-Israeli plot, and vowed that none of the suspects will ever be caught.

The STL is the world's only ad hoc tribunal able to try suspects in absentia and the only one set up to try terrorist suspects.