Syria says it will not use chemical weapons against its own people, but would do so against an external attack.
Syrian foreign ministry spokesman Jihad Makdissi said the weapons, stored and secured by the armed forces, would never be used “inside Syria”.
Earlier, the Arab League called on Syrian President Bashar al-Assad to step down, offering him safe passage.
Rebels have told the BBC’s Paul Wood, inside Syria, that they are encouraged by assassinations last week.
Four officials, including the defence minister and President Assad’s brother-in-law, were killed in an attack in Damascus on 18 July.
Rebels from the Free Syrian Army (FSA) told our correspondent, who is undercover with them near Damascus, that the deaths were a severe blow to the government.
They said the once-feared secret police were now a spent force, and the government was relying entirely on a weakened military.
‘External aggression’
“Any chemical or biological weapons will never be used, I repeat, will never be used in the Syrian crisis, no matter what the internal developments in this crisis are,” Mr Makdissi said, at a news conference broadcast on Syrian state TV.
“All varieties of these weapons are stored and secured by the Syrian armed forces and under its direct supervision, and will not be used unless Syria is subjected to external aggression.”
Syria has never officially confirmed it has chemical weapons. It is not a signatory to the Chemical Weapons Convention (CWC), which outlaws production.
The West and Israel were deeply worried that Syria might use its stocks of chemical weapons, says the BBC’s Jim Muir in neighbouring Lebanon.
On 16 July, the most senior Syrian politician to defect to the opposition told the BBC the government would not hesitate to use chemical weapons if it were cornered.
Nawaf al-Fares, Syria’s former ambassador to Iraq, said unconfirmed reports indicated such weapons might have already been used.
However, the opposition has not reported any use of chemical weapons.
Aleppo offensive
Meanwhile, Syrian government forces have retaken parts of Damascus that had fallen to the rebels.
Syrian state TV on Monday showed images of government forces going house-to-house and kicking down doors in Damascus, searching for any remaining rebel fighters.
Continued clashes are reported in the northern city of Aleppo.
Rebel launched a new offensive at the weekend, vowing to take the city completely and use it as a base for liberating the whole country.
Videos posted online on Monday showed jubilation by rebel fighters in the Sakhour district.
State TV played down the scale of the violence, saying troops were merely hunting down “terrorists”.
The most senior Turkish diplomat remaining in Syria, the consul in Aleppo, has been withdrawn for consultations.
Turkey, which closed its embassy in Damascus earlier this year, did not say whether the consul would return.
Diplomatic pressure
Arab League foreign ministers urged President Assad to resign rapidly, saying the opposition should form a transitional government.
EU foreign ministers have agreed to tighten sanctions and an arms embargo on the Syrian government.
EU member states will be required to send inspectors to board planes and ships on their territory believed to be carrying weapons or suspicious supplies to Damascus.
Britain and France have called for more EU aid to refugees from Syria.
UK Foreign Secretary William Hague said the EU should “step up our humanitarian assistance”.
He also called for more support for the opposition, “including helping them prepare for Syria after Assad”, Mr Hague said as he arrived for talks in Brussels.
Russian airline Aeroflot is to end flights to Damascus from 6 August, citing lack of demand.
On Sunday, the Syrian Observatory for Human Rights reported that at least 19,106 people had been killed since March 2011. The UN said in May that at least 10,000 people had been killed.
Syria blames the violence on foreign-backed “armed terrorist gangs”.
In June, the Syrian government reported that 6,947 Syrians had died, including at least 3,211 civilians and 2,566 security forces personnel.