Five days after their arrival, chemical weapons inspectors in Syria will be given access to an alleged attack site on Wednesday, Russia says.
The team has been in the country since Saturday but has been prohibited from entering the town of Douma.
US officials have raised concerns that Russia may have tampered with the site while inspectors were denied access.
Syria and its ally Russia deny responsibility for the attack, with Russia claiming that it was “staged”.
Activists on the ground in Syria say the attack killed more than 40 people and injured hundreds more sheltering from bombing in basements beneath the city. Video footage and witness testimony suggests that gas seeped down into the basements, suffocating the victims.
In response to the attack, the US, UK and France carried out targeted military strikes against Syrian chemical weapons facilities.
Russia and Syria had cited “pending security issues to be worked out” while inspectors from the Organisation for the Prohibition of Chemical Weapons were denied access to the site of the alleged attack, said Ahmet Uzumcu, the director general of the OPCW.
Syrian authorities instead offered the inspectors 22 witnesses who they said were at the location of the strike and could be brought to Damascus for interviews.
By the time inspectors arrive at the site on Wednesday, it will have been 11 days since the attack. They are expected to gather soil and other samples to help identify any chemicals used in the suspected attack.
US Ambassador Kenneth Ward said at an OPCW meeting at the Hague on Monday that there were fears Russian forces had tampered with the site during the delay, Reuters reported.
“It is our understanding the Russians may have visited the attack site,” Mr Ward said. “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.”
Russian Foreign Minister Sergei Lavrov denied any interference with the evidence. “I can guarantee that Russia has not tampered with the site,” he told the BBC.
He said evidence of the chemical attack was “based on media reports and social media” and was a “staged thing”.
In a separate incident on Tuesday, Syrian state media briefly reported that the country’s air defences had responded to a missile attack over the western city of Homs.
The US and Israel both denied carrying out any strikes, and later reports suggested the country’s air defence system may have been triggered by mistake.
What is said to have happened in Douma?
When the alleged chemical attack took place on 7 April, Douma, in the Eastern Ghouta region, was a final rebel stronghold near the capital Damascus – a suburban city battered by weeks of intense bombardment.
Two bombs filled with chemicals were reportedly dropped several hours apart on the town, targeting civilians hiding underground from regular bombs. Gas, heavier than air, seeps down into basements.
Syrian medical sources say men, women and children were found foaming at the mouth, with discoloured skin and cornea burns. US sources said they had obtained blood and urine samples from victims which had tested positive for chlorine and a nerve agent.
Catch up on the aftermath of the air strikes
- What was targeted?
- UK PM faces ‘almighty row’ over strikes
- Were the Syria air strikes legal?
- Reality Check: Does Russia always use a veto on Syria?
Late on Friday night in Washington, President Donald Trump announced that he had authorised strikes against Syrian military targets, in coordination with the UK and France.
US forces said 105 missiles were launched, and claimed the strikes had set back Syria’s chemical weapons programme by years.
Russia said 71 of the missiles were shot down by Syrian air defence systems.
One of the three sites hit was the Barzeh complex, which the US says was a centre for development, production and testing of chemical and biological weapons, although Syria denies this.
The other two were suspected chemical weapons facilities at Him Shinshar near Homs.
Syria chemical attack: Investigators allowed to visit site after delay}