They've waited two days to enter the site of the latest suspected chemical attack, where dozens of Syrian civilians were killed.
But despite having UN clearance, the Organization for the Prohibition of Chemical Weapons still can't enter the city of Douma, UK officials said.
The United Kingdom said Syria and Russia have blocked OPCW investigators from entering Douma, and the United States said Russia may have tampered with evidence at the site.
A representative of Russia's military said late Monday that OPCW inspectors would be permitted to travel to Douma on Wednesday.
"Wednesday is when we expect the arrival of the OPCW experts and Russia is not preventing this in any way," Igor Kirillov told journalists at The Hague.
Russia said its security forces would assist the OPCW inspectors.
Also Monday, Russia's envoy to the OPCW, Alexander Shulgin, released a statement accusing the the Syrian Civil Defense White Helmets of using US funds to fake the suspected chemical attack.
The OPCW team is tasked with determining whether banned substances were used in the attack, a claim that both Syria and its most powerful ally, Russia, have vehemently denied.
The team has been in Syria since Saturday, the same day the United States, United Kingdom and France carried out airstrikes against Syrian government targets in response to the April 7 attack in Douma.
Western leaders have blamed the Syrian government for the Douma attack, which killed about 75 people, including children, UK officials said.
Another 500 people were treated for symptoms consistent with chemical weapons exposure, the World Health Organization reported, citing its partners on the ground.
Syria's Deputy Foreign Minister Fayssal Mikdad said Monday his country was ready to facilitate the OPCW team in any way to carry out its mission, the state-run SANA news agency reported.
But Kremlin spokesman Dmitry Peskov did not respond directly to CNN's question on whether Russia had blocked the experts. Peskov said only that Russia was against "groundless" accusations about who was responsible for the attack.
Russian Deputy Foreign Minister Sergei Ryabkov said the OPCW team's arrival was hampered by the weekend airstrikes, state media RIA Novosti reported.
Last week's incident is just the latest in a series of suspected chemical attacks against civilians. The OPCW has recorded more than 390 allegations of chemical weapons use in Syria since its investigation began in 2014, said Peter Wilson, the UK's envoy to the OPCW.
May doubles down on blaming Syrian regime
British Prime Minister Theresa May and French President Emmanuel Macron have faced questions about their decision to join the United States in launching missiles at suspected chemical weapons sites in Syria.
But in a meeting with Parliament on Monday, May said it's clear Syrian President Bashar al-Assad's regime was behind the suspected chemical attack.
"From the intelligence I've seen, I'm in absolutely no doubt that the Syrian regime was responsible for the attack on Douma," she said.
"We must retain the right to commit our armed forces where it is right to do so without having a debate in Parliament."
Russian President Vladimir Putin and Iranian President Hassan Rouhani both called the missile strike an "illegal action" that "seriously damages the prospects for a political settlement in Syria."
Putin stressed that if "such actions, carried out in violation of the UN Charter, continue, it will inevitably lead to chaos in international relations," the Kremlin said.
The UK opposition leader, Jeremy Corbyn of the Labour Party, also questioned the strikes, saying Saturday that he believed the "action was legally questionable," and called on May to publish the UK attorney general's advice that approved the bombings.
As May and Macron face critics at home, their representatives to the United Nations will, with the United States, push for a resolution calling for an investigation into the alleged chemical weapons attack.
The renewed push for a UN-backed diplomatic solution follows a volatile emergency meeting of the UN Security Council, called by Moscow on Saturday, in which Russia condemned the strikes as a violation of international law.
War of words
The attack in Douma has also heightened tensions between Russia and Western powers. More than 100 Russian diplomats were expelled from more than 20 countries in March over the poisoning of a Russian ex-spy on British soil, which the United Kingdom and United States have blamed on the Russian government. Russia denies the accusations.
Russian officials last week claimed British intelligence agencies had helped stage the suspected chemical attack in Douma.
"Yet again, Russia is spreading conspiracy theories and misinformation designed to undermine the integrity of the OPCW's fact finding mission," said Wilson, the UK envoy to the OPCW.
"A significant body of information, including intelligence, indicates the Syrian regime is responsible for this latest attack."
The US envoy to the OPCW, Kenneth D. Ward, said he was concerned Russia may have "tampered with" the site of the Douma attack.
"This raises serious questions about the ability of the (team) to do its job," Ward said Monday.
But Russian Foreign Minister Sergey Lavrov told the BBC's "Hardtalk" show that there had been no Russian tampering. He said Russia sent experts to the site last week and found "no trace" of chemical weapons use.
Fresh Russia sanctions expected
US Ambassador to the UN Nikki Haley said Treasury Secretary Steven Mnuchin would announce new sanctions against Russia on Monday.
"If you look at what Russia is doing, they continue to be involved with all the wrong actors, whether their involvement in Ukraine, whether you look at how they are supporting Venezuela, whether you look in Syria and their way of propping up Assad and working with Iran," Haley said on "Fox News Sunday."
She said Trump has "talked to our allies and said they need to step up more. They need to do more. And it shouldn't just be us doing it. ... If we leave, when we leave, it will be because we know that everything is moving forward."
Despite their unified stance to eliminate chemical weapons from Syria, the United States, United Kingdom and France haven't given a consistent strategy on how to handle the Syrian conflict long term.