BELGRADE U.N. Secretary-General Ban Ki-moon said on Monday it would be "reprehensible" for Syria to use chemical weapons after Damascus warned it could deploy them if it felt threatened by foreign intervention.
Syria acknowledged for the first time on Monday 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".
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.
Ban said U.N. Undersecretary 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 U.N. 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.
(Reporting by Aleksandar Vasovic; Writing by Matt Robinson; Editing by Andrew Osborn)