In April 2017, more than 80 people died in a chemical weapons attack on Khan Sheikhoun - another Syrian town held by anti-government forces.
The United Nations Security Council will meet twice on Monday following rival requests by Russian Federation and the United States.
The determination was made with some degree of confidence, said the sources, who stopped short of assigning blame for the attack on Saturday, which medical aid groups said killed dozens of people in the town of Douma. Slamming Syrian president Bashar al-Assad calling him an animal and calling out Russian president by name.
The war in Syria is in the news following a suspected chemical weapons attack, in which at least 70 people have died.
Last year, one factor in Trump's decision to bomb Syria was televised images of dead children.
Mr Johnson added: "Should it be confirmed that the regime has used chemical weapons again, it would be yet another appalling example of the Assad regime's brutality and blatant disregard for both the Syrian people and its legal obligations not to use chemical weapons".
The pope urged political and military leaders to "choose the other way: the way of negotiations, the only one that can bring a peace that is not the peace of death and destruction".
"We are ready, once Douma is freed from militants, to immediately send Russian specialists in radiation, chemical and biological defence to collect data that will confirm these claims are fabricated", he said, quoted by news agencies.
"There's nothing left for civilians and fighters".
Later Sunday, dozens of buses entered Douma to take detainees released by Jaish al-Islam to government-held territory, according to Syrian state TV.
The military statement came after President Vladimir Putin warned against speculation and "provocation" over the chemical weapons attack in Douma.
"When any country chooses to ignore the global norms against the use of chemical weapons, they have made a deeply immoral decision with worldwide implications - implications that the United States can not ignore".
Fielding questions at the White House, Trump press secretary Sarah Sanders said it would be "outrageous" to say that Trump's recent announcement that he intends to remove all USA forces from Syria in the coming months had emboldened Assad.
The Syrian Foreign Ministry responded to the rebels' claims of chemical weapons use by the Syrian army in Douma district near Damascus as "premeditated pretexts", according to state news agency SANA.
Presidents Emmanuel Macron and Donald Trump agreed during a telephone call on April 8 that chemical agents were used in the deadly attack, the Elysee Palace said in a statement.
Hundreds of people have been killed and thousands more wounded in the offensive on Eastern Ghouta, which was once home to an estimated 400,000.
Saudi Arabia voiced "deep concern" and condemned the chemical attack.
Over the past few years, the government has been facing Western accusations of using chemical weapons against the civilians.
That attack prompted the United States to launch 59 Tomahawk cruise missiles at a Syrian airbase.
Trump has declared his intent to withdraw US troops from Syria in the coming months, despite resistance from many of his advisers.