The first group of experts from the global chemical weapons watchdog have travelled to Syria to begin investigating an alleged nerve agent attack, the agency has said.

The Organisation for the Prohibition of Chemical Weapons said the fact-finding mission would arrive in Syria on Thursday and start work in Douma, eastern Ghouta, on Saturday.

Bashar al-Assad’s regime is accused of killing more than 40 people, many of them children, in a chemical weapons attack on the town, a rebel enclave near Damascus.

Syria and its ally Russia have claimed reports of the attack were fabricated by rebels and rescue workers.

But Western powers are convinced regime forces were responsible and are considering military action against Syria.

Syria’s UN ambassador said the regime would facilitate a visit by international chemical weapons inspectors at “any point they want”. Bashar Ja’afari said OPCW’s team were being given visas and claimed any delay or “disruption of their visit” would be as a result of “political pressure” from Western countries.

US president Donald Trump has pledged to respond ”forcefully” to the alleged chemical attack and said America had “a lot of options militarily”.

“We can’t let atrocities like we all witnessed... we can’t let that happen in our world... especially when we’re able to because of the power of the United States, the power of our country, we’re able to stop it,” he said.

Theresa May joined Mr Trump in calling for a response if Syrian regime’s latest use of chemical weapons was confirmed. She summoned ministers to a meeting at Downing Street on Thursday to discuss possible action.

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Meanwhile, the Mr Trump was holding his own meetings in the US.

Fears of confrontation between the West and Russia, Syria‘s key ally, have been running high since the president said on Wednesday missiles “will be coming” and lambasted Moscow for standing by Syrian president Bashar al-Assad.

On Thursday, however, he wrote in a tweet: “Never said when an attack on Syria would take place. Could be very soon or not so soon at all!”

Later, he added: “We’re having a number of meetings today, we’ll see what happens. Now we have to make some... decisions, so they’ll be made fairly soon.”

French President Emmanuel Macron said France had proof the Syrian government carried out the attack near Damascus, which aid groups have said killed dozens of civilians, and will decide whether to strike back when all the necessary information has been gathered.