BEIRUT (AP) — 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 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.

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."

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Syria is believed to have nerve agents as well as mustard gas, Scud missiles capable of delivering these 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.

Makdissi did not discuss last week's bombing claimed by the rebels that killed four top Syrian security officials, but assured journalists that the situation was under control, despite reports of clashes throughout the country and especially in the major cities of Aleppo and the capital Damascus.

"Yes, there were clashes on certain streets in certain neighborhoods, but the security situation is now much better. Everyone is feeling reassured," he said. "We are not happy about this, but this is an emergency situation and it will not last more than a day or two and the situation will return to normal."

Security forces appeared to show more government control in videos posted online by activists Monday. Some of the clips show Syrian militia sweeping through Damascus neighborhoods once held by rebels, kicking down doors and searching houses in mop up operations against the fighters that had managed to hold parts of the capital for much of last week.

It was a different story in Aleppo, however, where the Britain-based Syria Observatory reported fierce fighting in a string of neighborhoods, including Sakhour and Hanano, in the northeast of Syria's largest city.

Several videos posted by activists showed rebels battling regime tanks in the narrow streets of Sakhour. In one case, a tank on fire rumbles past after being hit by rebels as a man escapes from the flaming turret. Other videos showed cheering rebels celebrating around destroyed tanks, even driving around one they had captured.

The Observatory said many people fled these neighborhoods in the subsequent lulls in the fighting. The Associated Press could not independently verify the videos posted by the activists.

Aleppo, Syria's biggest city with about 3 million residents, has been the focus of rebel assaults by a newly formed alliance of opposition forces called the Brigade of Unification. The group said Sunday it was launching an operation to take the city.

Even as the government appeared to be reasserting control in the capital after the weeklong rebel assault, the Arab League offered Syrian President Bashar Assad and his family a "safe exit" if he steps down.

"This request comes from all the ... Arab states: Step aside," said Qatari Prime Minister Hamid bin Jassim Al Thani at an Arab League foreign ministers meeting in Doha, Qatar, that concluded at dawn Monday. He urged Syria to form a temporary transitional government to plan for a possible post-Assad era. Makdissi dismissed the offer as "flagrant interventionism."

The Arab League has already suspended Syria's membership and it is doubtful that Assad will pay much attention to their calls. He ignored a similar request to step down in exchange for asylum by Tunisian President Moncef Marzouki last February.

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AP security writer Kimberly Dozier in Washington and Associated Press writer Albert Aji in Damascus, Syria, contributed to this report.