BEIRUT - President Bashar al-Assad's feared brother Maher lost a leg in a bomb attack on the Syrian leader's security cabinet a month ago, sources said on Thursday, in a severe blow to one of the main military commanders fighting a 17-month-old insurgency. The attack on a meeting of Assad's security chiefs in Damascus on July 18 killed four members of the president's inner circle, including his brother-in-law, and emboldened the rebels to take their fight to the capital for the first time. Maher has not been seen in public since the bombing, while Assad himself has restricted appearances to recorded clips broadcast on television, leading to speculation about the effectiveness of the leadership as the rebellion grows. Maher, a close associate of the president, has acquired a fearsome reputation as the commander of the Syrian army's Republican Guard and 4th Division, elite formations largely composed of troops from the Assads' minority Alawite sect, whose loyalty can be relied on in the fight against the rebels. "We heard that he (Maher al-Assad) lost one of his legs during the explosion, but don't know any more," a Western diplomat told Reuters. A Gulf source confirmed the report: "He lost one of his legs. The news is true." The disclosure of Maher's injury came as fears grew that the conflict that has already claimed the lives of at least 18,000 people in Syria was starting to spill over its borders into a region already torn by sectarian divisions. Gulf Arab states told their citizens to leave Lebanon after a Lebanese Shi'ite clan kidnapped more than 20 people in Beirut and initially threatened to seize more Arab nationals. The gunmen said a Turkish hostage, whose country is a key backer of Syria's mainly Sunni Muslim insurgency, would be the first to die if one of their kinsmen held by Syrian rebels in Damascus were killed. The powerful Meqdad family is seeking to put pressure on rebels to release clan member Hassan al-Meqdad, who has been held by the Free Syrian Army (FSA) for two days. An earlier threat by the kidnappers to seize Saudis, Turks and Qataris to secure the release of their kinsman bore ominous echoes of Lebanon's own civil war - and Arab governments lost no time in urging visitors to leave Beirut's popular summer tourist haunts. Saudi Arabia, the United Arab Emirates, Qatar, Kuwait and Bahrain all told their nationals to leave at once. Some nations have already begun flying their citizens home. "The snowball will grow," warned Hatem al-Meqdad, a senior member of the Meqdad family who said his brother Hassan was detained by the FSA two days ago.