BEIRUT - A UN-backed tribunal set up to try suspects in the 2005 assassination of Lebanese statesman Rafik al-Hariri on Thursday rejected charges by Lebanon's Hezbollah armed group that its work is politically motivated. "Experience of other international tribunals has shown that the results of the work of such institutions speak for themselves and contradict the unsubstantiated allegations of hostile interference," Fatima Issawi, spokeswoman for the tribunal, told Reuters in written answers to emailed questions. "We are convinced that this will also happen in the case of the Special Tribunal for Lebanon (STL)." Saudi Arabia's King Abdullah and Syrian President Bashar al-Assad are due to pay an unprecedented joint visit to Beirut on Friday to try to calm a political storm over the tribunal. This month Hezbollah leader Sayyed Hassan Nasrallah assailed the court, which is based in The Hague, as an "Israeli project" after saying he had received word that it planned to indict members of his group in connection with Hariri's killing. He denied that any Hezbollah members were involved. Early reports by UN investigators implicated Syrian and Lebanese security agencies. Syria says it had no hand in the February 14 seafront bombing in Beirut that killed Hariri and 22 others. The assassination provoked an international furor led by the United States, France and Saudi Arabia that prompted Syria to end its 29-year military presence in Lebanon in April 2005 and led to the establishment of the special tribunal. Asked about future indictments, Issawi said: "It would be quite unhelpful to add to the existing speculations. The Office of the Prosecutor will issue an indictment when it is ready." The tribunal has indicted no one since it was set up by the UN Security Council in May 2007. Last year it ordered the release of four pro-Syrian Lebanese generals jailed for four years without charge. Lebanese authorities had detained them at the request of a former U.N. investigator in 2005.