The Russian military said on Monday its medics had examined patients in a hospital in Douma and had found no traces of a chemical attack, Interfax news agency reported.
The United States has asked the UN Security Council to vote Tuesday on its proposal to set up an inquiry to investigate chemical weapons attacks in Syria, but the measure could face a veto from Russian Federation, diplomats said.
Russian Federation and Syria have blamed Israel for an attack on a Syrian military airport on Monday that reportedly killed at least 14 people.
"The seriousness of the recent allegations requires a thorough investigation using impartial, independent and professional expertise", he said.
President Trump has promised a forceful US response to Saturday's incident in the Damascus suburb of Douma.
He said the resolution was trying to recreate the old expert body, whose extension Moscow blocked in November.
Nebenzia rejected a United States call for the world body to set up a probe to identify the perpetrators of what rescuers called a "poisonous chlorine gas attack" late Saturday in the enclave of Douma, near Damascus.
Boris Johnson tweeted that Russia's veto was "hugely disappointing".
Regime ally Moscow said it would propose a United Nations resolution on a "transparent and honest" investigation with the OPCW's involvement.
Trump promised on Monday forceful action in response to the attack that killed at least 60 people and injured more than 1,000, according to a Syrian relief group.
Russia is the Syrian regime's only real supporter, and there are Russian troops in Syria supporting President Assad's troops.
If such threats from the US and its allies, France and Britain, are meant to stop Damascus from fighting terrorism, "they are delusional", the statement also said. "I am not like other administrations, where they say, 'We are going to do this in four weeks.' It doesn't work that way".
Britain says Foreign Secretary Boris Johnson spoke to Acting U.S. Secretary of State John Sullivan by phone and the two "agreed that, based on current media reports and reports from those on the ground, this attack bore hallmarks of previous chemical weapons attacks by the Assad regime".
The Daily Telegraph newspaper said May had ordered British submarines to move within missile range of Syria in readiness for strikes against the Syrian military that could begin as early as Thursday night.
Global bodies led by the Organisation for the Prohibition of Chemical Weapons (OPCW) were trying to establish exactly what happened in Douma, a rebel-held town in the eastern Ghouta district. "The team is preparing to deploy to Syria shortly".
The crisis was discussed Monday at an urgent UN Security Council meeting, while the USA also circulated a draft resolution for a new independent inquiry of chemical weapons attacks in Syria.
They said the United Kingdom and the USA must work closely together and with global partners to ensure those responsible are held to account. "We just have to hope that Moscow will not overreact when the strikes come".