A Lebanese army colonel has been detained on suspicion of spying for Israel, security sources said on Tuesday.
The sources said the colonel was arrested last week and was being questioned about links to Israeli spy agencies.
Lebanon is holding up to 30 suspects in what security sources say is a widening investigation into espionage for Israel.
At least 21 suspects have already been charged, some in absentia, and several have confessed, the authorities say. Israel has not commented on the arrests.
Lebanese officials have displayed what they say is sophisticated communications equipment and other gadgets found in the homes or offices of some of the suspects.
Lebanon says at least two spies fled to Israel last week and has demanded Israel hand them back.
Sayyed Hassan Nasrallah, leader of the Iranian- and Syrian-backed Lebanese Islamist group Hizbullah, last week called for the death penalty for all suspects convicted of spying for Israel.
Senior Lebanese security officials say the arrests have dealt a major blow to Israel's spying networks in Lebanon.
They say many of the suspects played key roles in identifying Hizbullah targets that were bombed during a 34-day war between Israel and the Shiite group in 2006.
Other suspects have been charged with monitoring senior Hezbollah officials and at least one is alleged to have played a role in the 2004 assassination of a commander of the group.