BEIRUT (Reuters) -- A leader of the Lebanese militant group Fatah al-Islam, Abdel Ghani Jawhar, has died in Syria while trying to plant a bomb for rebels fighting President Bashar Assad, Lebanese security sources said on Wednesday.

Jawhar's involvement with Syria's armed opposition, who have been trying to topple Assad for more than a year, will embarrass Assad's opponents who have denied Syrian government assertions that it is fighting an "armed terrorist" revolt.

The security sources said that Jawhar died on Friday in the town of Qusair, a few miles into Syria from the Lebanese border and a hotbed for anti-Assad sentiment.

Jawhar used to be a member of the Muslim Brotherhood, but joined the more radical Fatah al-Islam group two years ago.

In 2007, Fatah al-Islam fighters clashed with the Lebanese army for 15 weeks at the Nahr al-Bared Palestinian refugee camp in north Lebanon, after Lebanese security forces surrounded the camp in search of militants from the group.

At least 430 people were killed in the fighting, including 170 soldiers and 220 militants.

The group, designated by the United States Department of State as terrorist organization, was formed in 2006 in Nahr al-Bared.