The Pentagon said Thursday a "preponderance of the evidence" indicates there were chemical weapons, including "elements" of sarin nerve gas, at the three Syrian sites bombed by the USA and its allies last week.
It is believed that the Syrian President Bashar al-Assad used a highly toxic chemical agent on civilians.
They have risked a confrontation with Moscow, the Syrian regime's top ally, with President Vladimir Putin warning that fresh attacks would spark "chaos", while Washington vowed economic sanctions against Russian Federation rather than further military action.
No. The organization's job is to establish whether chemical weapons were used, not who used them.
Worldwide chemical weapons inspectors have not yet managed to enter the Syrian town of Douma - and it's not clear when they will be able to visit the town, which was the site of a suspected chemical weapons attack on April 7.
Earlier on Tuesday, Syrian state television reported that the experts from the Organisation for the Prohibition of Chemical Weapons had entered Douma.
White House Press Secretary Sarah Huckabee Sanders said in a statement, "We are considering additional sanctions on Russian Federation and a decision will be made in the near future".
A child receiving oxygen through respirators following an alleged poison gas attack in the rebel-held town of Douma, near Damascus, Syria.
Chemical experts will be given access to the alleged attack site in Douma on Wednesday, but USA officials have raised concerns that Russian Federation may have "tampered" with evidence.
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Russian Foreign Ministry spokeswoman Maria Zakharova dismissed allegations that Russia was trying to hamper the OPCW inspectors, saying Moscow strongly supports their mission to Douma.
Survivors reached by the Associated Press said they were overwhelmed by the smell of chlorine on the night of April 7. But it said no missiles landed inside the bases.
Johnson said the missiles were launched to degrade and deter the Assad regime's ability to deploy chemical weapons.
Russian Federation denied the claims, saying parts of Douma still needed to be de-mined and said the Organization for the Prohibition of Chemical Weapons (OPCW) inspectors would enter on Wednesday.
United Nations spokesman Stephane Dujarric said Wednesday that Staffan de Mistura embarked on the consultations following a meeting Monday with Secretary-General Antonio Guterres on the sidelines of the Arab League summit in Riyadh, Saudi Arabia "and in view of current tensions".
The use of chemical weapons violates the most fundamental part of worldwide law.
He also said that despite Syria's commitment in 2013 to destroying its chemical arsenal - and Russia's promise to supervise to it - "the Assad Regime and Russian Federation has made a complete mockery of that pledge".
" 'We've had over a week now where people, lots of different people, we don't know who may have traipsed in and out of that, ' he says".
The assault on eastern Ghouta began in February and ended in government victory on Saturday when rebels withdrew from the town. Saudi Arabia is also a member of the USA -led coalition battling IS militants.