"Many dead, including women and children, in mindless CHEMICAL attack in Syria," Trump tweeted.
"Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!" the U.S. president wrote.
Trump also pinned blame for the situation on former US President Barack Obama's Syria policies: "If President Obama had crossed his stated Red Line In The Sand, the Syrian disaster would have ended long ago! Animal Assad would have been history!"
Syrian activist groups on Saturday said toxic gas inside barrel bombs dropped from helicopters over a rebel-held city in Syria killed dozens of civilians and wounded scores more. Syrian state news said an "official source" denied the allegations.
In the early hours of Monday morning, Syria State TV reported that "several missiles" had been fired at the T4 air base in Homs, in the country's west, adding that it was "likely an attack by the US." The Pentagon issued a statement Sunday denying it was conducting air strikes in the region.
The United Nations Security Council will hold an emergency meeting Monday on the alleged chemical weapons use, the US mission to the United Nations announced. Russia has requested a separate Security Council meeting later in the day, two UN diplomats told CNN.