International inspectors are trying to visit the site of a suspected gas attack which brought US-led missile strikes on Syria and heightened the diplomatic confrontation between the West and President Bashar al-Assad's main ally Russia.
Russian President Vladimir Putin said on Sunday more Western attacks on Syria would bring chaos to world affairs, and Washington prepared to increase pressure on Russia with new economic sanctions.
Moscow also condemned the Western states for refusing to wait for the findings of the Organisation for the Prohibition of Chemical Weapons (OPCW) inspection team on the alleged attack before launching the strikes.
But the US envoy to the global watchdog said on Monday Russia may have tampered with the site of the incident on April 7 in Douma outside of Damascus.
"It is long overdue that this council condemns the Syrian government for its reign of chemical terror and demands international accountability of those responsible for these heinous acts," US Ambassador Kenneth Ward said in comments seen by Reuters.
In London, British Prime Minister Theresa May was facing criticism over her decision to bypass parliament and take part in the air strikes against Syria.
The United States, France and Britain launched 105 missiles targeting what the Pentagon said were three chemical weapons facilities in Syria in retaliation for the suspected poison gas attack in Douma.
The Western countries blame Mr Assad for the Douma attack, which a Syrian medical relief group said killed dozens of people. The Syrian government and its Russian ally deny involvement.
Inspectors for the Hague-based OPCW met Syrian Deputy Foreign Minister Faisal Mekdad in the presence of Russian officers and a senior Syrian security official in Damascus for about three hours on Sunday.
The inspectors were due on Monday to attempt to visit Douma, but the British delegation to the OPCW said they had not yet been granted access, citing the agency's director general.
Douma, which lies in the eastern Goutha district on the outskirts of the capital, was one of the last bastions near Damascus of rebels fighting to topple Mr Assad, and the alleged attack took place amid a ferocious government offensive.
In the aftermath, the remnants of the rebel army evacuated, handing Mr Assad one of the biggest victories in a war that has killed about half a million people.
The Western allies have said the aim was to prevent the further use of chemical weapons, not to intervene in the civil war or topple Mr Assad.
The US ambassador to the UN, Nikki Haley, said on Sunday the US would announce new economic sanctions aimed at companies dealing with equipment related to Mr Assad's alleged use of chemical weapons.