Rescuers and medics in the town of Douma say more than 40 people died after the alleged poison gas attack in the last rebel-held pocket of the one-time opposition stronghold of Eastern Ghouta.
"Any confirmed use of chemical weapons, by any party to the conflict and under any circumstances, is abhorrent and a clear violation of international law," Secretary-General Antonio Guterres said in a statement.
"The seriousness of the recent allegations requires a thorough investigation using impartial, independent and professional expertise," he said.
Guterres reaffirmed his full support for the Organisation for the Prohibition of Chemical Weapons (OPCW) and its fact-finding mission which "should be granted full access, without any restrictions or impediments to perform its activities," Guterres added.
US President Donald Trump was poised Tuesday to decide on possible military action against the Syrian regime, after vowing to respond "forcefully" to the latest alleged chemical atrocity in the country's civil war despite strong warnings from Damascus-ally Russia.
The United States, backed by Britain and France, has said it is ready to act with or without support from the United Nations, where the Security Council was to vote as early as Tuesday on rival US and Russian proposals to probe chemical attacks in Syria.
Trump met his cabinet and then dined with top generals Monday, telling reporters "we have a lot of options militarily and we'll be letting you know pretty soon... probably after the fact."
Washington and Moscow set out starkly different positions at a stormy emergency session at the UN Security Council, where Russia's ambassador Vassily Nebenzia warned that the possibility of US military action was "very, very dangerous."
Russia, a major Syrian ally, said Tuesday it will propose a UN resolution to investigate the alleged attack.
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May to speak to Trump
British Prime Minister Theresa May said she would talk to Trump later on Tuesday about the suspected chemical attack.
"I'll be continuing to talk with our allies and partners as I have done, speaking to President Macron this morning, and I'll be speaking to President Trump later today," she told reporters in Cambridgeshire in eastern England.
When asked whether Britain would join the United States if Washington decided on further military action in Syria, May declined to answer the question directly but said: "We believe that those responsible should be held to account."
May said she would chair a meeting of Britain's National Security Council later on Tuesday.
"This attack that took place in Douma is a barbaric attack. Obviously we are working urgently with our allies and partners to assess what has happened on the ground," she said.
Russian Foreign Minister Sergei Lavrov said Moscow will propose a UN resolution to investigate alleged chemical weapons attacks in Syria, after Russia rejected a similar move from the United States.
The rival proposals from Moscow and Washington put the Security Council on course for a clash that could trigger vetoes of both measures.
Lavrov said Russia would propose a "transparent and honest" investigation with the involvement of the Organisation for the Prohibition of Chemical Weapons at the Security Council on Tuesday.
