The US State Department said Mr Johnson and his Washington counterpart had discussed "potential further steps the US and United Kingdom governments might take in coordination with other partners". Vice president Mike Pence will travel in his place.
The Syrian government has denied the accusations. Lyons called that response "small" and recommended Mr. Trump think "through whether he wants a tactical response, something quick on the ground, or he's got to think through the strategy hopefully involving our allies". Mrs May did not answer directly when asked if Britain would join the U.S. if it chose to take a military response, saying instead: "We believe that those responsible should be held to account".
After the second Russian measure was defeated, Nebenzia addressed US Ambassador Nikki Haley directly, appealing to the United States to refrain from military action.
But Russia says it can not support the proposal because it contains "unacceptable elements".
Worldwide chemical weapons experts are expected to visit Douma to investigate the attack, but Russian Federation and Assad's government have said there was no evidence a gas attack had taken place.
President Donald Trump warned on Monday that there would be a "big price to pay" for the suspected attack.
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The Britain-based Syrian Observatory for Human Rights said that Syrian government troops were on a 72-hour alert and were fortifying their positions.
It was not immediately clear whether the announcement would delay or prevent a USA strike in Syria.
As he began an evening meeting with military leaders at the White House, Trump promised to "make a decision tonight or very shortly thereafter".
Haley said Russian Federation was to blame for the council's failure to take action on Syria, now in its 8th year of war. "Probably after the fact".
Rescuers and medics in Douma say more than 40 people died after the suspected poison gas attack in the last rebel-held pocket of the one-time opposition stronghold of Eastern Ghouta.
Pressure is mounting on the British Prime Minister to support President Donald Trump, who on Monday promised "forceful action" would be taken by the U.S. against the Assad regime.
Trump was reacting to a warning from Russian Federation on Tuesday that any US missiles fired at Syria over the deadly assault on a rebel enclave would be shot down and the launch sites targeted.