President Barack Obama has warned Syria's President Bashar al-Assad that his government will be held accountable if it uses chemical weapons.
He was speaking after Damascus said they would not be deployed inside Syria but would be against foreign attack.
UN Secretary General Ban Ki-moon said that any thought of using chemical weapons would be "reprehensible".
The sharp international response came hours after Syrian foreign ministry spokesman Jihad Makdissi gave Damascus's first implicit acknowledgement that a chemical weapons stockpile existed.
The US and Israel have expressed concern about the fate of Syria's arms, with Israeli PM Benyamin Netanyahu speaking of a "great threat" of weapons sites falling into the hands of Lebanon's Shia Islamist group Hezbollah, allied to President Assad.
The State Department in Washington said any possible use of such weapons would be "completely unacceptable".
Fierce fighting has taken place in Aleppo, where rebels claim to have captured parts of the city.
There are also unconfirmed reports from activists in Homs that security forces are threatening to storm the city's central prison.