The statement made no specific reference to military action.
Earlier in the day, Bashar al-Jaafari, Syrian ambassador to the United States, said in NY that two investigating teams from the OPCW were scheduled to arrive in Syria within the next 24 hours. He said he has been talking regularly this week with U.S. President Donald Trump about the most effective response.
The rising tension over the Douma attack demonstrates the volatile nature of the Syrian civil war, which started in March 2011 as an anti-Assad uprising but is now a proxy conflict involving a number of world and regional powers and a myriad of insurgent groups.
Moscow on Thursday called on the West to "seriously consider" the consequences of threats against Syria after the U.S. and France said they would respond to an alleged chemical attack.
In the call, the two leaders had agreed that the regime of Syrian President Bashar al-Assad "had established a pattern of risky behaviour in relation to the use of chemical weapons", Downing Street said.
British Prime Minister Theresa May was due to hold an emergency cabinet meeting amid speculation she will support United States action against the Syrian regime.
Officials also said that the US has compiled intelligence from the USA and other countries, including images, that indicate the Syrian government was behind the weekend attack.
He also said the proof shows it was carried out by the regime of Syrian president Bashar al-Assad.
Later, May's office said she had spoken with Trump by telephone, and the two had agreed it was vital to challenge Assad's use of chemical weapons, and that they would continue to work closely together to do so.
Twenty-one percent of the respondents said they did not know if it was necessary to hold a parliamentary vote on whether London should engage in the military actions against Syria. May isn't legally required to do that, though it has become conventional since the 2003 invasion of Iraq for lawmakers to be given the chance to vote before British forces are deployed.
Britain has launched air strikes against Islamic State militants in Syria, but not against the country's government.
British lawmakers voted down taking military action against Damascus in 2013, in what was widely viewed as an assertion of parliamentary sovereignty on the use of force.
A YouGov poll in The Times conducted this week found that 43 percent of voters oppose strikes in Syria, with 34 percent unsure and only 22 percent supportive.
"Cabinet agreed on the need to take action to alleviate humanitarian distress and to deter the further use of chemical weapons..."
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.