"I don't rule out anything right now," Mattis said while hosting Sheikh Tamim Bin Hamad Al-Thani, the emir of Qatar, at the Pentagon.
"The first thing we have to look at is why are chemical weapons still being used at all when Russia was the framework guarantor of removing all chemical weapons, and so working with our allies and partners from NATO to Qatar and elsewhere we are going to address this issue," Mattis continued.
President Donald Trump on Sunday warned Syria and its two closest allies, Russia and Iran, of a "big price to pay" following the attack, and slammed Assad as an "animal" on Twitter.
At least 70 people died in a suspected chemical attack in Douma, the last rebel-held town in Syria's Eastern Ghouta. It said the deaths were likely to rise.