BEIRUT, Lebanon — The Syrian regime threatened Monday to use its chemical and biological weapons in case of a foreign attack, in its first ever acknowledgement that it possesses weapons of mass destruction.

Foreign Ministry spokesman Jihad Makdissi vowed, however, that Damascus would not use its unconventional arms against its own citizens. The announcement comes as Syria faces international isolation, a tenacious rebellion that has left at least 19,000 people dead and threats by Israel to invade to prevent such weapons from falling into rebel hands.

Syria's decision to reveal the long suspected existence of its chemical weapons suggests a desperate regime deeply shaken by an increasingly bold rebellion that has scored a string of successes in the past week, including a stunning bomb attack that killed four high-level security officials, the capture of several border crossings and sustained offensives on the regime strongholds of Damascus and Aleppo.