* Chemical weapons experts' access to site blocked
* West says no more strikes planned just now
* Allied leaders face scrutiny at home
* More sanctions mooted by US, EU (Adds detail)
By Laila Bassam and Anthony Deutsch
DAMASCUS/THE HAGUE, April 16 (Reuters) - The United States accused Russia on Monday of blocking international inspectors from reaching the site of a suspected poison gas attack in Syria and said Russians or Syrians may have tampered with evidence on the ground.
Moscow immediately denied the charge and blamed delays on retaliatory U.S.-led missile strikes on Syria at the weekend.
In the fraught aftermath of the suspected attack in Douma and the West's response, Washington also prepared to increase pressure on Russia, Syrian President Bashar al-Assad's main ally, with new economic sanctions. European Union foreign ministers threatened similar measures.
And in London and Paris, British Prime Minister Theresa May and French President Emmanuel Macron faced criticism from political opponents over their decisions to take part in the air strikes against Syria.
Syria and Russia deny unleashing poison gas during their offensive on Douma this month, which ended with the recapture of the town that had been the last rebel stronghold near the capital Damascus.
Relief organisations say dozens of men, women and children were killed. Footage of young victims foaming at the mouth and weeping in agony helped to thrust Syria's civil war -- in which half a million people have been killed in the past seven years -- to the forefront of world concern again.
Inspectors from the Hague-based Organization for the Prohibition of Chemical Weapons (OPCW) travelled to Syria last week to inspect the site, but have yet to gain access to Douma, which is now under government control after the rebels withdrew.
"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," he said. His comments at the closed-door meeting were obtained by Reuters.
Russian Foreign Minister Sergei Lavrov denied that Moscow had interfered with any evidence. "I can guarantee that Russia has not tampered with the site," he told the BBC.
Britain's delegation to the OPCW accused Russia and the Assad government of stopping inspectors from reaching Douma.
"Unfettered access is essential," the British delegation said in a statement. "Russia and Syria must cooperate."
The team aims to collect samples, interview witnesses and document evidence to determine whether banned toxic munitions were used, although it is not permitted to assign blame for the attack.
Britain's Ambassador Peter Wilson said in The Hague that the United Nations had cleared the inspectors to go but they had been unable to reach Douma because Syria and Russia had been unable to guarantee their safety.
Moscow blamed the delay on the air strikes, in which the United States, France and Britain targeted what the Pentagon said were three chemical weapons facilities.
"We called for an objective investigation. This was at the very beginning after this information [of the attack] appeared. Therefore allegations of this towards Russia are groundless," Kremlin spokesman Dmitry Peskov said.
U.S. envoy Ward also condemned the Syrian government for what he called its "reign of chemical terror".
The inspectors 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.
OPCW inspectors have been attacked on two previous missions to the sites of chemical weapons attacks in Syria.
Syrian flags flew in Douma on Monday, security forces stood on street corners and Russian military police patrolled the streets. State aid trucks handed out bread, rice and pasta to people who had lived under siege for years.
A government-organised media tour did not include the building where, according to rescue workers and medics who were in town at the time, dozens of people were killed by poison gas.
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.
Medical aid groups and the White Helmets rescue organisation have said such statements - already aired on state television in recent days - were made under duress.
The U.S.-led strikes did nothing to alter the strategic balance or dent Assad's supremacy and 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 Assad.
British Foreign Secretary Boris Johnson made this clear on Monday as he arrived at a meeting of EU foreign ministers in Luxembourg, telling reporters: "I'm afraid the Syrian war will go on in its horrible, miserable way. But it was the world saying that we've had enough of the use of chemical weapons."
The 28 ministers endorsed the missile strikes and considered steps to deepen Assad's isolation.
"The European Union will continue to consider further restrictive measures against Syria as long as the repression continues," they said in a statement after their talks.
The U.S. ambassador to the United Nations, Nikki Haley, said on Sunday the United States would announce new economic sanctions aimed at companies dealing with equipment related to Assad's alleged use of chemical weapons.
The Western leaders also faced scrutiny at home over their actions.
Britain's May will make a statement to parliament on Monday on her decision and will repeat her assertion that Assad's forces were highly likely responsible for the attack.
The allies could not wait "to alleviate further humanitarian suffering caused by chemical weapons attacks", according to excerpts of her speech.
But she will be questioned over why she did not seek parliamentary approval for the action, a decision that she and her ministers say was driven by the need to act quickly.
Jeremy Corbyn, leader of the main opposition Labour Party, has questioned the legal basis for Britain's involvement.
Britain has said there are no plans for future strikes against Syria, but Johnson warned Assad that all options would be considered if chemical weapons were used against Syrians again.
In France, the conservatives, the far-left and the far-right have all criticised the strikes.
Prime Minister Edouard Philippe on Monday will respond to their criticism in a debate in parliament on Monday afternoon. The French Constitution bars presidents from going to parliament and President Emmanuel Macron will therefore not be questioned by law-makers.
(Reporting by Leila Bassam in Damascus, Anthony Deutsch in Amsterdam, Jack Stubbs and Andrey Ostroukh in Moscow, Jeff Mason, Susan Cornwell and Joel Schectman in Washington, Michelle Nichols in New York, Samia Nakhoul, Tom Perry, Ellen Francis and Angus McDowall in Beirut, Kinda Makieh in Barzeh, Syria, Elizabeth Piper, Michael Holden and Guy Faulconbridge in London, Laurence Frost, Michel Rose and Ingrid Melander in Paris, Writing by Angus MacSwan; Editing by Janet Lawrence)
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