In a series of tweets, Trump wrote, “President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay.”
Trump said Syrian troops have surrounded the site of the attack and are now allowing humanitarian aid to reach victims.
The current president also blamed his predecessor for the attack, saying if Barack Obama had lived up to his threats, “Animal Assad would have been history!”
An alleged gas attack killed at least 40 people in the eastern suburbs of Damascus, local responders said on Sunday. Syrian state media, meanwhile, reported that rebels there have agreed to give up their last foothold in the area.
First responders and a relief organization said they found families suffocated in their homes and shelters in the besieged town of Douma, with foam on their mouths.
The reports, which started circulating late Saturday, could not be independently verified, and the government denied allegations it had used chemical weapon in its assault on the town.
*With files from the Associated Press