Moscow meanwhile warned Washington on Sunday of carrying out a "military intervention on fabricated pretexts" in Syria.
At Monday's emergency meeting, several Security Council members believed that the Syrian government was responsible for Saturday's chemical attack in Douma, while Russian Federation claimed that the whole incident was a fabrication.
He said the USA and its allies want to know why chemical weapons use continues after Russian Federation guaranteed the removal of all such weapons from Syria.
Syrian state media has denied the government had any involvement in the suspected attack.
"President Trump has indicated that the United States will decide a response shortly... Important decisions are being weighed, even as we speak".
It seems very likely that the Trump administration will take some sort of military action against Syria, but just what and when remains unclear.
"We are very concerned that a thing like that can happen".
Mrs May sidestepped a question on whether Parliament would be recalled from recess to debate any military intervention.
Acting Secretary of State John Sullivan spoke by phone Monday with British Foreign Secretary Boris Johnson.
"Recent developments carry more than ever before" the danger of triggering the multiple "fault lines in the Middle East", which could lead to "absolutely devastating consequences, which are hard to even imagine", the official said, underscoring the possibility that the war in Syria may spiral into a larger global security crisis. We need more than a reaction.
In February, when Russia moved its SU-57 stealth fighter jets into Syria, a Russian politician appeared to be referring to Israel when he said the presence of the fighter jets was sending a political message "to aircraft from neighboring states which periodically fly into Syrian airspace uninvited". "Everybody's going to pay a price, he will, everybody will".
Trump said there was little question that Syria was responsible for the apparent weekend attack, although the government of President Bashar Assad denied it.
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".
Democratic Sen. Ben Cardin of Maryland, a senior member on Foreign Relations, said the U.S. needs to go the diplomatic route and work with the worldwide community to build a case against Syrian President Bashar al-Assad.
The Syrian government and its ally Russian Federation have denied involvement in the attack. It said the victims have been transferred to Tehran, and that funerals will be held in the victims' hometowns across the country.
Mr Trump had warned of a "big price to pay" following the reports of a poison gas attack on the Syrian town of Douma which killed dozens of people, including children.
The attack comes nearly exactly a year after a chemical atrocity in the northern Syrian town of Khan Sheikhoun killed dozens of people. In April previous year, the USA fired cruise missiles at a Syrian airbase after a Sarin nerve agent attack on the opposition-held town of Khan Sheikhoun killed more than 80 people.
The Syrian-American Medical Society said more than 500 people were brought to medical centres in Douma, in the Eastern Ghouta region, near the capital Damascus, with symptoms "indicative of exposure to a chemical agent".