The tribunal, in a statement, said the prosecution was seeking to add "a new count to the indictment of 'criminal association." Amid speculation over the details of the filing, the STL noted "contents of the amended indictment remain confidential."
The tribunal noted the appeals chamber wasn't called on to define the crime of "criminal association" when it was asked to define terrorism and conspiracy in its case.
The STL is investigating the 2005 assassination of Hariri, a former Lebanese prime minister. Four members of Hezbollah are accused of playing a role in the assassination, something the Shiite movement denies.
The tribunal in July released the names of suspects -- Salim Jamil Ayyash, Mustafa Amine Badreddine, Hussein Hassan Ainessi and Assad Hassan Sabra -- who are wanted for the bombing attack in which a total of 22 people died.
The tribunal announced that it assigned permanent counsel to the four men. They'll be tried in absentia this year.