The State Department said Washington would "respond quickly and appropriately" if Assad was found to have launched a chemical weapon attack.
The U.S. has resettled only 44 Syrian refugees since October.
Senior officials also suggest Russia must have known about the latest gas attack ahead of time.
The senator said Rex Tillerson may have signaled that the U.S. would steer clear of conflict in the country.
Donald Trump is considering taking action in Syria, in response to the a chemical weapons attack that killed 84 civilians in the war-torn country
The death toll is likely to rise.
According to Human Rights Watch, the attacks were part of the overall military strategy to retake the city.
The probe shows the decision to use toxic weapons came from the very top.