A prominent Hezbollah commander, Muhammad Ali Yunis, was killed by unknown gunmen on Sunday morning in southern Lebanon, Iranian and Lebanese media reported.
The Iranian semi-official Fars news agency reported that the slain commander was "responsible for tracking spies and collaborators".
It also said that so far there is "no additional information on how he was killed".
Some Lebanese media outlets close to the Hezbollah group said that Yunis was pursuing a suspect when he was ambushed by three cars in the Zoutar area.
Opposition Lebanese media said Israeli intelligence agency Mossad is behind the assassination. The claims have not been verified.
No official statement has yet been issued by Hezbollah on the incident.