"All acts of terrorism are much more complicated than war crimes, crimes against humanity and genocide," Antonio Cassese of the UN-backed Special Tribunal for Lebanon said during his first visit to Beirut. "We cannot set deadlines," he added.

Cassese, who arrived in Lebanon on Monday on a weeklong visit, said that unlike other crimes, political assassinations were harder to crack given the layers of secrecy involved.

"Terrorist acts involve secret cells. There's no clear chain of command or hierarchy," he said. "Most of the time those involved will not confess because they risk being killed by their accomplices."

Cassese and his deputy, Ralph Riachy, briefed senior officials including President Michel Suleiman on the tribunal's progress during their visit.

They did not meet Prime Minister Saad Hariri, son of the murdered premier, or Defense Minister Elias Murr "because of the personal link they have with cases that may fall under the tribunal's jurisdiction," Cassese's office said in a statement before his visit.

Murr was also a minister in Rafik Hariri's Cabinet.

The Hague-based tribunal was set up by a UN Security Council resolution in 2007 to try suspects in the murder of Hariri.