France said it would take action against the government of Syrian President Bashar Al Assad if evidence emerges that it was responsible for a chemical attack on a rebel-held town in Eastern Ghouta.
"If the red line has been crossed, there will be a response," government spokesman Benjamin Griveaux said.
He said that intelligence shared by President Emmanuel Macron and his American counterpart, President Donald Trump, "in theory confirms the use of chemical weapons”.
Syria said on Tuesday that it has invited an international chemical weapons watchdog to send a fact-finding mission into the country to investigate the suspected poison gas attack.
The Foreign Ministry said Syria will help the Organization for the Prohibition of Chemical Weapons to investigate the alleged attack, which opposition activists say killed 40 people over the weekend.
Also on Tuesday the White House announced the cancellation of Mr Trump's first trip to Latin America, scheduled for later this week, saying he wants to "oversee the American response to Syria."
The US, backed by Britain and France, has said it was ready to act with or without support from the United Nations, where the Security Council was to vote on Tuesday on rival US and Russian proposals to investigate chemical attacks in Syria.
Mr Trump met his cabinet and then dined with a senior general on Monday, telling reporters: "We have a lot of options militarily and we'll be letting you know pretty soon … probably after the fact".
Rescuers and medics in Douma said that at least 48 people were killed after a suspected poison gas attack in the last rebel-held pocket of Syria.
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The UN voiced alarm on Tuesday at "spiralling new displacement" from Eastern Ghouta after more than 133,000 people were estimated to have fled in four weeks, and where UN aid agencies still do not have access.
“We are aware of screening taking place as civilians are leaving Eastern Ghouta, but as you know we are not part of current evacuation deals or its implementation," said Andrej Mahecic, spokesman of the UN refugee agency UNHCR.
The Kremlin on Tuesday accused Washington of refusing to face up to reality over the alleged chemical attack.
"You see the unconstructive position that some countries including the US have taken. They are a priori refusing to face reality," Kremlin spokesman Dmitry Peskov said.
He said that "none of them is talking about the need for an unbiased investigation" and said that this limited the diplomatic options for Russia, but that it would continue "active work on the diplomatic front”.
Syrians in Douma 'left suffocating amid air strike gas attack'
Meanwhile, a huge blast tore through a multi-story building in the rebel-held city of Idlib, in northern Syria.
The cause of the explosion was not apparent, but the Britain-based Syrian Observatory for Human Rights said 13 people were killed and another 80 injured.
The Organisation for the Prohibition of Chemical Weapons said it was investigating the Douma attack allegations, but that so far only a "preliminary analysis" had taken place.
Syrian regime forces have carried out an offensive against Ghouta since February 18 that has killed more than 1,700 civilians and cornered rebels in their last holdout of Douma.
After capturing most of Ghouta, Syria and Russia secured two negotiated withdrawals last month that allowed 46,000 rebels and civilians to leave.
Following fraught negotiations and a government bombing campaign, state media on Sunday announced a deal for Jaish Al Islam to leave Douma within 48 hours and release hostages.
A group of detainees arrived in Damascus overnight Monday into Tuesday after being freed by Jaish Al Islam, state news agency Sana said.
In a parallel operation, 65 buses carrying fighters and civilians left the outskirts of Ghouta and headed to northern Syria, according to the Syrian Observatory for Human Rights.
