Pretty obvious Putin knows the chemical weapons are on the Syrian airbase as Russian troops have shared it with the Syrians for a while now, Putin is complicit in the recent gas attack.
"On Thursday, Secretary of State Rex Tillerson said that Russia had failed to carry out a 2013 agreement to secure Syrian chemical weapons, adding that Moscow was either complicit or incompetent in its ability to uphold that deal.
Tillerson said that the U.S. had a high degree of confidence that sarin nerve gas had been used in Tuesday's chemical attack in northern Syria."
Trump launches attack on Syria with 59 tomahawk missiles