Chemical weapons were used by Syria's Assad regime last Saturday on Douma, a holdout east of Damascus later quit by rebels, Macron told France's TF1 television on Thursday.

"We have proof that chemical weapons were used last week, at least chlorine, and that they were used by the regime of Bashar Assad," said the French leader.

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Macron said one of his aims in Syria was to "remove the regime's chemical attack capabilities," but added that he wanted to avoid "an escalation."

French and US officials were "working very closely, and we will have decisions to take, at the time we choose, when we consider it most useful and most effective."

Asked if and when Syrian facilities would be targeted, Macron replied: "When we decide, and we will have to verify all the information."

UK backs united front

Government ministers in Britain agreed at an emergency meeting in London on the need for a response to the chemical weapons attack.

Ministers "agreed on the need to take action to alleviate humanitarian distress and to deter the further use of chemical weapons by the Assad regime," Prime Minister Theresa May's office said, adding that they "agreed the prime minister should continue to work with allies in the United States and France to coordinate an international response."

Separately, US officials obtained urine and blood samples from victims in Douma that tested positive for chlorine and a nerve agent, MSNBC reported. 

A team from the world's chemical arms watchdog, the Organization for the Prohibition of Chemical Weapons (OPCW), was expected to arrive in Syria on Thursday and Friday begin a fact-finding mission in Douma. Any results from the investigation could take weeks or months. 

Syrian, Russian denials

Syria and its ally Russia have both denied involvement amid worldwide outrage over Saturday's incident.

In Berlin, Chancellor Angela Merkel said Germany would not participate in any military strike on Syria.

Germany wanted, however, to ensure "that all efforts are made to demonstrate that this attack with chemical weapons is not acceptable," she said.

Hundreds treated

The World Health Organization (WHO) has said 500 patients were treated for symptoms of exposure to toxic chemicals.

The White Helmets, an organization working inside Syria, claimed that 43 people were killed in the attack on Douma.

Since Saturday, several thousand rebels and civilians have left Douma for opposition-held territory in the north of Syria.

Those departed included Issam Buwaydani, the head of the rebel militia Jaysh al-Islam, according to the Britain-based Syrian Observatory for Human Rights.

cw,ipj/ng (AFP, dpa)