U.S. President Barack Obama has warned Syria it would be a "tragic mistake" to use its chemical and biological weapons after the regime acknowledged for the first time it possesses weapons of mass destruction.

In a speech on Monday to the Veterans of Foreign Wars at their annual convention in Reno, Nev., Obama warned Syria would be held accountable if such weapons were used.

"Given the regime's stockpiles of chemical weapons, we will continue to make it clear to [President Bashar] Assad and those around him that the world is watching and that they will be held accountable by the international community and the United States should they make the tragic mistake of using those weapons," Obama said.

Foreign Ministry spokesman Jihad Makdissi stressed, 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 attack to prevent such weapons from falling into rebel hands.

A Free Syrian Army soldier reacts during clashes with Syrian government troops in Aleppo in this image made from amateur video. (Ugarit News via Associated Press)

The timing, however, of Syria's decision to reveal the long suspected existence of its chemical weapons suggests a desperate regime deeply shaken by an increasingly bold revolt 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.

"No chemical or biological weapons will ever be used, and I repeat, will never be used, during the crisis in Syria no matter what the developments inside Syria," Makdissi said in news conference broadcast on Syrian state TV. "All of these types of weapons are in storage and under security and the direct supervision of the Syrian armed forces and will never be used unless Syria is exposed to external aggression."

The Syrian government later tried to back off from the announcement, sending journalists an amendment to the prepared statement read out by Makdissi adding the phrase "if any," in attempts to return to their previous position of neither confirming or denying the existence of unconventional weapons.

The regime subsequently blasted foreign media outlets for taking its remarks out of context and focusing on the announcement of chemical weapons instead of its attempt to "respond to a media campaign aimed at preparing international opinion for foreign intervention into Syria under the false pretext that it was going to use weapons of mass destruction inside the country."

Mustard gas, missiles

Syria is believed to have nerve agents as well as mustard gas, Scud missiles capable of delivering the lethal chemicals and a variety of advanced conventional arms, including anti-tank rockets and late-model portable anti-aircraft missiles.

Israel has said it fears that chaos following Assad's fall could allow the Jewish state's enemies to access Syria's chemical weapons, and has not ruled out military intervention to prevent this from happening.

A senior U.S. intelligence official said Friday the Syrians have moved chemical weapons material from the country's north, where the fighting was fiercest, apparently to both secure it, and to consolidate it, which U.S. officials considered a responsible step.

But there has also been a disturbing rise in activity at the installations, so the U.S. intelligence community is intensifying its monitoring efforts to track the weapons and try to figure out whether the Syrians are trying to use them, the official said on condition of anonymity to discuss the still-evolving investigation.