Syria's President, Bashar al Assad, said a possible attack on his country could trigger regional instability. "You don't have to be partners with animals that kill their people and take pleasure in it!" he said.
In April past year Trump ordered Tomahawk strikes on the Shayrat Airbase in response to a similar chemical weapons attack on rebel-held Khan Sheikhun.
He went on to state, "We have not yet made any decision to launch military attacks into Syria. Could be very soon or not so soon at all!"
Syria and Russian Federation have blamed Israel for the attack, which has similarly been confirmed by the U.S. Israel has neither confirmed nor denied performing the operation.
Mr. Mattis' remarks at a House Armed Services Committee hearing followed a series of Trump tweets this week that initially indicated he was committed to bombing Syria but later suggested he was awaiting further advice and assessment.
In response, Maria Zakharova, Russia's foreign ministry spokesperson, said allegations of the use of chemical weapons in Douma were false and could not be used as a pretext to undertake any military action against Syria.More news: First Rohingya family repatriated in Myanmar despite United Nations concerns
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In Paris, France's Emmanuel Macron upped the pressure on Moscow by stating he had "proof" that the Assad's regime had used chemical weapons, and vowing a response "at a time of our choosing".
The Organisation for the Prohibition of Chemical Weapons, based in the Netherlands, announced it was sending a fact-finding team to the site of the attack outside Damascus, and it was due to arrive Saturday.
When asked whether the American blood will be spilt in case Russian lives are lost, Russian blood is spilt, Zakharova said, "I do not really want to even think about this because you know, I just want to remind you, myself and everybody who are watching this interview that we are living in 2018". Syria and Russian Federation denied the attack.
Earlier, the US President Donald Trump has tweeted that Russian Federation should "get ready" for the firing of missiles at Syria as the US continues to affirm that the Syrian government was behind an alleged chemical attack on Saturday.
Separately on Thursday, German Chancellor Angela Merkel said Berlin would not join any military strikes against Syria, but supports efforts to show the use of chemical weapons is unacceptable.
At stake in Syria is the potential for confrontation, if not outright conflict, between the USA and Russian Federation, former Cold War foes whose relations have deteriorated in recent years over Moscow's intervention in Ukraine, its interference in the 2016 US presidential election and its support for Syrian President Bashar Assad.