He said he had "at this point not ordered military engagement".
But he added: "There would be enormous consequences if we start seeing movement on the chemical weapons front or the use of chemical weapons."
Earlier the new UN special envoy to Syria faced criticism for refusing to say whether

President Assad must quit.
President Obama, speaking to reporters at a White House briefing, said the deployment or use of biological weapons would widen the conflict in the region.
He said: "It doesn't just include Syria. It would concern allies in the region, including Israel, and it would concern us."
He warned President Bashar al-Assad and "other players on the ground" about the use or movement of such weapons.
He said: "A red line for us is [if] we see a whole bunch of chemical weapons moving around, or being utilised. That would change my calculus."
Syria holds the world's fourth-largest stockpile of chemical weapons. Last month a Syrian foreign ministry spokesman said the weapons would never be deployed inside Syria.
However, the BBC's Kim Ghattas in Washington says the US has seen unconfirmed reports recently that the Syrian authorities have been moving the country's chemical arms stockpile.