“When you watch babies and children being gassed and suffer under barrel bombs, you are instantaneously moved to action,” White House spokesman Sean Spicer told a news briefing. “I think this president has made it very clear that if those actions were to continue, further action will definitely be considered by the United States.”

Adding barrel bombs to the list of verboten weapons significantly widens the sort of military activity by Assad that would warrant action.

Previously, only chemical attacks, or attacks using weapons of mass destruction, could do so.

Spicer did not elaborate on what a potential U.S. response could include, but said additional actions are a possibility.

“The sight of people being gassed and blown away by barrel bombs ensures that if we see this kind of action again, we hold open the possibility of future action,” he said.

"If you gas a baby, if you put a barrel bomb in to innocent people, I think you ​will see a response from this president," he added.

His comments are part of a stark departure in policy for the Trump administration, which had adopted a much softer line on Assad before a deadly chemical attack in Idlib province prompted Washington to carry out military action in Syria.

In all the U.S. fired 59 cruise missiles at the Shayrat Airfield from two U.S. destroyers in the Eastern Mediterranean last Thursday, targeting the base’s refueling infrastructure, and destroying about 20 percent of the fixed-wing aircraft there.

“The Syrian government has lost the ability to refuel or rearm aircraft at Shayrat Airfield and at this point, use of the runway is of idle military interest,” Defense Secretary James Mattis said in a statement on Monday.

“The Syrian government would be ill-advised ever again to use chemical weapons,” he added.

Russia, Assad regime's principal backer, regards the ​strike on Syria as an "act of aggression" against a sovereign state in violation of international law under a "far-fetched pretext".

Anadolu Agency