Western officials are also insisting on the moral argument for action, saying Syrian President Bashar Assad must be stopped from using particularly cruel and outlawed weapons.
Russian Federation is a major backer of the government of Syrian President Bashar al-Assad in the country's civil war, and strongly opposes the US claim that Syrian government forces attacked a rebel-held town near Damascus on Saturday with chemical weapons. "I just want to remind you one more time it's 2018, it's not the middle of the 20th century", she said, adding that in a time of instant communications there's no reason "why people who are in charge of our security and global peace can't talk".
Japan will only support U.S. President Donald Trump's "resolve" to prevent any further use of chemical weapons in Syria, the sources said.
Another US official familiar with how the US intelligence is unfolding regarding the chemical attack tells CNN that the initial intelligence assessment - that a chemical agent was used - is based on viewing the videos and comparing the shown physical effects with what is known about both chlorine and nerve agents.
During his testimony, Mattis spoke about a possible strike against Syria. "Could be very soon or not so soon at all!"
Later on Thursday, French President Emmanuel Macron said his country had proof that the Syrian government launched the gas attacks.More news: Lawyer sets himself on fire to protest climate change
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"I will tell you that, first of all, did a chemical weapons attack happen?"
"It's too bad that the world puts us in a position like that", said Trump, as Defense Secretary James Mattis headed to the West Wing to present options for a retaliatory strike.
US President Donald Trump is weighing all options on the table with regard to Syria as he holds the Syrian regime and Russian Federation responsible for the latest chemical weapons attack, the White House said today, adding that no final decision has been taken yet on the military response.
British Prime Minister Theresa May called an emergency cabinet meeting for Thursday, with reports indicating that she has already ordered submarines to move within missile range of Syria.
"The Prohibition of Chemical Weapons (OPCW) confirms that the OPCW Fact-Finding Mission (FFM) team is on its way to Syria and will start its work as of Saturday 14 April, 2018", the watchdog said in a statement on Thursday. "Numerous, large-scale attacks on another country without Congressional authorisation are unconstitutional, and they push the U.S. closer to what could be an interminable, all-out conflict in Syria".
He pointed out that former U.S. President Barack Obama also threatened to take military actions against Syria over similar allegations, but did not carry out his threat.