The tribunal, which was inaugurated in 2009, will continue its work until March 2018, said a UN statement on Friday.
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Four Hezbollah members went on trial in absentia before the tribunal a year ago for the 2005 car bombing that killed billionaire Hariri and 22 others, including a suicide bomber.
A fifth suspect, also an alleged Hezbollah member, has been indicted over the crime.
Ban expressed UN support for the work of the tribunal based in The Hague.