Syria acknowledged for the first time that it possessed chemical and biological weapons, saying they would not be used against rebels but could be used if the country faced "external aggression".
International pressure on President Bashar al-Assad has escalated dramatically in the last week, alongside a rebel offensive in the two biggest cities and a bomb attack which killed four members of his inner circle in Damascus.
Defying Arab foreign ministers who on Sunday offered Assad a "safe exit" if he stepped down, the Syrian leader has launched fierce counter-offensives, reflecting his determination to keep power as a 16-month uprising enters its most violent phase.
Foreign Ministry spokesman Jihad Makdissi said the army would not use chemical weapons to crush rebels but could use them against forces from outside the country.
On the same day that Syria admitted it had chemical and biological weapons, President Assad's forces were fighting back against the rebels in Syria's second biggest city Aleppo.
International Editor John Irvine reports.
Damascus has not signed a 1992 international convention that bans the use, production or stockpiling of chemical weapons, but officials in the past had denied it had any stockpiles. Western countries expressed immediate alarm.
US President Barack Obama said that Syrian President Bashar al-Assad would be held accountable if he made the "tragic mistake" of using his stockpile of chemical weapons.
UN Secretary-General Ban Ki-moon said it would be "reprehensible" for Syria to use chemical weapons after Damascus warned it could deploy them if it felt threatened by foreign intervention.
Ban said he was concerned about the possibility that Syria "may be tempted" to use chemical weapons.
"It would be reprehensible if anyone in Syria would use weapons of mass destruction," he told reporters during a visit to Serbia.
As violence in the 16-month-old uprising against President Bashar al-Assad escalates, insurgents say they fear his forces will resort to non-conventional weapons as they try to claw back rebel gains across the country.
Foreign Secretary William Hague called Syria's threat to use chemical weapons against foreign intervention "unacceptable"
Speaking to reporters at a European Union foreign ministers' meeting in Brussels, Hague said: "This is typical of the complete illusion of this regime, that they are the victims of external aggression.
"What is actually happening is their own people are rising up against a brutal police state ... and in any case it is unacceptable to say that they would use chemical weapons under any circumstances."
Ban said UN Under-secretary General for Peacekeeping Operations Herve Ladsous and top U.N. military adviser Gen. Babacar Gaye were leaving for Syria on Monday to assess the situation on the ground.
He said Gaye would take over from Gen. Robert Mood as the head of the UN monitoring mission in Syria after the U.N. Security Council voted on Friday to extend its mandate for 30 days under a faltering peace plan drawn up by international envoy Kofi Annan.
International pressure on Assad has intensified in the last week with a rebel offensive in the two biggest cities and a bomb attack which killed four members of his inner circle in Damascus.