On Sunday, US ambassador to the United Nations Nikki Haley said new sanctions would be announced on Monday by Treasury Secretary Steve Mnuchin.
"It is our understanding the Russians may have visited the attack site", U.S. Ambassador Kenneth Ward said at an OPCW meeting in The Hague on Monday. "It is our concern that they may have tampered with it with the intent of thwarting the efforts of the OPCW fact-finding mission to conduct an effective investigation".
The comments came during a closed-door meeting at the Organisation for the Prohibition of Chemical Weapons, convened after an April 7 attack in the town of Douma, outside the Syrian capital, in which dozens of people were allegedly killed with poison gas.
He also urged Syria to "end this charade", "immediately cease all" such attacks and "immediately declare and dismantle all aspects of its chemical weapons program".
It strongly denied interfering with the work of inspectors attempting to reach the site of the atrocity which the United Kingdom and Western allies have concluded was perpetrated by the regime of Moscow's ally Bashar Assad.
"I can not be impolite with the heads of other states but you quoted the leaders of France and the United Kingdom and USA and, frankly speaking, all the evidence they quoted was based on media reports and social media".
Guterres said he has asked Syrian special envoy Staffan de Mistura to come to NY as soon as possible to consult "on the most effective way to accelerate the political process".
America is expected to place sanctions on Russian Federation on Monday in retaliation for Moscow's support of Syria's chemical weapons programme - but White House officials have cautioned that no decision has yet been taken.
The OPCW inspectors will not assign blame for attacks.
The team was still not been granted safe passage on Monday, CNN quoted British envoy to the OPCW, Peter Wilson, as saying.
While he declared success, the Pentagon said the pummelling of three chemical-related facilities left enough others intact to enable the Assad government to use banned weapons against civilians if it chooses.
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What was targeted on Saturday?
"The Syrian regime has reportedly been attempting to hide the evidence by searching evacuees from Douma to ensure samples are not being smuggled from this area and a wider operation to hide the facts of the attack is under way, supported by the Russians", she told MPs.
The French president gave a wide-ranging TV interview on Sunday night saying he had persuaded US President Donald Trump not to pull troops out of Syria and instead commit "for the long term".
"We are considering additional sanctions on Russian Federation and a decision will be made in the near future", White House spokeswoman Sarah Sanders said in a statement.
Doctors at the hospital where suspected victims were treated told reporters on the tour that none of the patients that night had suffered chemical weapons injuries - they were asphyxiated by dust and smoke in a bombardment.
The proposed sanctions follow USA -led airstrikes against Syria's chemical weapons capability and President Trump's promise that Assad and his allies, namely Moscow, would pay a "big price" for enabling the use of chemical weapons.
"The US mission has not changed - the president has been clear that he wants US forces to come home as quickly as possible".
Within the European Union, which is due to hold an worldwide donor conference for Syria next week, most governments now agree that Assad can not continue as president for peace talks to succeed.
Although officials said the singular target was Assad's chemical weapons capability, his air force, including helicopters he allegedly has used to drop chemical weapons on civilians, were spared.
Addressing the French parliament, Prime Minister Edouard Philippe said Saturday's operation had not been the "prelude to a war".