Reports of a suspected gas attack began circulating minutes later.
The Organization for the Prohibition of Chemical Weapons, an global chemical weapons watchdog organization said Thursday that its team is en route to Syria and will begin work Saturday to investigate the attack.
Russian Federation has warned the United States against carrying out a "military intervention on fabricated pretexts", and has accused the White Helmets civil defense organization in Syria of staging a fake chemical weapons attack in Douma. Due to the extreme risk of war escalating into a nuclear conflict between the world's two greatest nuclear powers, and the fact that "the Russian conventional forces just aren't as strong as the USA forces", such a fight "would not be a good outcome for Russia". "They are a guarantee of the observance of law and order in the town", the defence ministry said in a statement.
Al-Jaafari was quoted by SANA news agency as saying that the Syrian government was ready to facilitate the teams' entry into any point in Douma, Eastern Ghouta's largest town.
France says "there will be a response" if the "red line" of chemical weapons use has been crossed.
"This is a recent development, but we think it's a potential for a clash there", Mattis told reporters.
Later he attempted to clarify his threat, saying that he had never specified when the United States would conduct an attack on Syria.
Mattis also promised communication to the United States congressional leadership prior to any attack on Syria.
Earlier on Thursday, the Russian Defense Ministry confirmed that Syrian government troops had gained full control of the Douma enclave in Eastern Ghouta, 10km from Damascus. "Any mission, any time, anywhere - we are ready to go".
Yevtushenko heads the Russian center for reconciliation of the warring parties in Syria.
Russian Federation is already moving its chips to the possibility of a United States air strike against its ally in the Middle East, the regime of Bashar al-Assad.
"Putin is not interested in a shooting war with the West", Gorenburg said.
Russian Federation recently said reports of the chemical attacks are untrue and "bogus".
The news comes days after Douma was the scene of a horrifying attack, in which dozens of Syrians died from suffocation. Syrian activists, rescuers and medics said the attack in Douma killed at least 40 people, with families found suffocated in their houses and shelters.
"Russia vows to shoot down any and all missiles fired at Syria", Trump tweeted Wednesday.More news: Networking is given by television celebrity Khloe Kardashian
"We're maintaining that we have a number of options and all of those options are still on the table".
Earlier, Russia's United Nations envoy, Vassily Nebenzia cautioned the U.S.to "refrain from the plans that you're now developing", saying Washington would "bear responsibility" for any "illegal military adventure".
Douma is allegedly the site of a chemical attack which was reported by rebel-linked activists, including the controversial White Helmets group.
That may prove too little, too late.
Russian Federation has deployed air defense systems in Syria, including its sophisticated S-400 long-range surface-to-air missile system.
Mattis said the president's National Security Council would be meeting later today to "take forward the various options" for Trump.
The council has already failed to agree on a response to the attack in three votes and has been deadlocked throughout the Syrian civil war.
However, French President Emmanuel Macron claimed that Paris had "the proof" that the Syrian government had used chemical weapons, including chlorine, "last week".
Prime Minister Theresa May held a special cabinet meeting to weigh whether Britain should join in any possible military action.
Amid the talk of military action, the Kremlin, a close ally of Assad, countered that more "serious approaches" were needed to combat the crisis.
National security experts anxious about whether strikes would actually serve to deter Assad.
Trump suggested Monday he had little doubt that Syria was to blame, but neither he nor other administration officials have produced hard evidence.
Elsewhere in her remarks, Zakharova underlined the need for "an immediate and objective investigation" into the suspected chemical weapons attack on the Syrian town of Douma on April 7.
Russian Federation says threats by the U.S. and France to launch a military aggression against Syria violate the UN Charter, calling on the West to "seriously consider" the consequences of such intimidation.