PARIS (AP) — Western powers are preparing a tough response if Syrian President Bashar Assad’s regime deploys chemical or biological weapons in its civil war, key European officials warned Monday.

Syria’s leadership has said the country, which is believed to have nerve agents as well as mustard gas and Scud missiles capable of delivering them, could use chemical or biological weapons if it were attacked from outside.

“Our response would be massive,” French Foreign Minister Laurent Fabius told RMC radio.