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France will retaliate against the regime of Syrian President Bashar al-Assad if evidence emerges that it was behind a recent suspected chlorine gas attack in rebel-held eastern Ghouta, government spokesman Benjamin Griveaux said on Tuesday.

"If the red line has been crossed, there will be a response," he told Europe 1 radio, adding that intelligence shared by President Emmanuel Macron and US President Donald Trump "in theory confirms the use of chemical weapons".

In a phone call on Monday night, the two leaders again discussed the latest alleged chemical attack in Syria's civil war, in the city of Douma near Damascus on Saturday.

Rescuers and medics in Douma say at least 60 people died after the suspected poison gas attack in the last rebel-held pocket of the one-time opposition stronghold of Eastern Ghouta.

France has repeatedly warned that evidence of the use of chemical weapons in Syria is a "red line" that would prompt French strikes on Syrian regime forces.

Trump for his part said Monday that "we have a lot of options militarily and we'll be letting you know pretty soon... probably after the fact".

The US leader is set to lay out his response to the attack shortly, with sources reporting that military action is likely. Trump is expected to demand France and the UK join with US forces in whatever military action is decided upon.

See in pictures: Syrians stand in solidarity with Douma

In April last year, Trump launched a cruise missile strike against a Syrian air base within 72 hours of a chemical attack UN monitors later pinned on Assad's regime.

The UN Security Council, gathered in New York, is expected to vote on Tuesday on rival US and Russian proposals to probe chemical attacks in Syria, with Russia expected to veto any investigation proposed by the US.

Moscow has denounced the claims as "fabrications", with its UN envoy warning Monday the possibility of military action was "very, very dangerous". 

Following the French and US statements, the UK is under pressure to issue a verdict on potential military action in response to the chemical attacks.

UK Prime Minister Theresa May said on Monday that the Syrian regime "must be held to account" if it and its backers are responsible for the attack.

"If confirmed this is yet another example of the Assad regime's brutality and brazen disregard for its own people and for its legal obligation not to use these weapons," May told reporters in Copenhagen following talks with Danish Prime Minister Lars Lokke Rasmussen. 

"If they're found to be responsible, the regime and its backers, including Russia, must be held to account.

"The UK utterly condemns the use of chemical weapons in any circumstances and we must urgently establish what happened on Saturday," May said.

British Foreign Secretary Boris Johnson spoke to his French counterpart on Monday and called for a "strong and robust international response" following the suspected chlorine attack.

Although Johnson did not explicitly apportion blame for the attack, the UK shares the same intelligence as the US and France, which points concretely to the Russian and Iranian-backed Syrian regime as the perpetrators.

Agencies contributed to this report.

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