The Syrian government and Russian Federation, which backs it, strongly oppose the claims, which they say are fabricated.
"We can not exclude any possibilities, unfortunately", Mr Nebenzia told reporters after a private meeting of the UN Security Council in NY.
"China is anxious about the possibility of escalation of tension", Geng said.
Half a world away in New York, Russia's United Nations ambassador warned the priority in Syria was to avert US-led strikes that could lead to a unsafe confrontation between the world's two preeminent nuclear powers.
Military brass and senior national security officials huddled at the White House for 90 minutes on Thursday afternoon to consider their options.
"We are continuing to assess intelligence and are engaged in conversations with our partners and allies".
By supporting Assad, Russia's actions are "prejudicial to our security" and will let the Islamic State group "re-establish itself" - something that "we believe we need to take action to defend", said Pierce.
Trump has remained in heavy consultation with France and the United Kingdom since the suspected chemical attack, which killed more than 40 people.
Davis, who in 2013 voted against taking military action against Syrian President Bashar al-Assad's government, said that his decision then was based on a lack of clear evidence and a lack of a clear plan.
On Thursday, US officials were quoted as saying that samples from victims had tested positive for chlorine and a nerve agent.
Foreign Minister Jean-Yves Le Drian said Thursday that President Emmanuel Macron would decide whether to launch an attack over the "non-respect of the worldwide convention against chemical weapons", which is a "red line" for France.
U.S. Defense Secretary James Mattis said Thursday President Donald Trump has yet to decide on whether he will pursue military action in Syria.
"They agreed to keep working closely together on the worldwide response", the statement concluded.
What has Trump said about the attack?
In April previous year Trump ordered Tomahawk strikes on the Shayrat Airbase in response to a similar chemical weapons attack on rebel-held Khan Sheikhun.
Mr Trump, who has cancelled a planned trip overseas, has been canvassing support for strikes from the leaders of France and the UK.
President Donald Trump warned Russian Federation on Wednesday to "get ready" for a missile attack on its ally Syria, but tweeted Thursday that it may come "very soon or not so soon at all!" It "could be very soon or not so soon at all", he said.
"We have not yet made any decision to launch military attacks into Syria", Mattis told lawmakers on the House Armed Services committee.
But she says, "No final decision has been made".
Later, he told reporters a decision will "be made fairly soon".
As tensions mount between the USA and Russian Federation over a possible U.S. strike, the Kremlin said a crisis communications link with the United States meant to avoid an accidental clash over Syria was being used by both sides.
Russian Federation has described the reports of a chemical attack as a "provocation" created to justify Western intervention.
Turkish President Recep Tayyip Erdogan says he will discuss ways of ending "the chemical massacre" in Syria during a telephone call with Russia's President Vladimir Putin.
He added that Russian Federation would "keep all its global obligations in full".
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