The Syrian state news agency SANA earlier reported that the worldwide experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) had entered Douma to begin their investigation of whether chemical agents were used as a weapon.
The warning came as the USA ambassador to the OPCW, Ken Ward, voiced fears that Moscow might already have "tampered with" evidence at the site.
The punitive attacks early on Saturday were launched before a fact-finding team from the OPCW was able to enter Douma and begin its fieldwork.
The Western countries rushed to blame the Douma incident on the Syrian government, but Damascus strongly rejected the accusation as fabrications meant to halt advances made by pro-government forces against terrorists.
Increasing regional jitters, Syrian anti-aircraft defenses shot down missiles fired at the air base of Shayrat in Homs province late on Monday and at another base northeast of the capital, Damascus, Syrian state television and pro-Iranian Hezbollah media said.
The Associated Press, during a government-organized visit Monday to Douma, spoke to survivors and witnesses who described being hit by gas.
"It was in our national interest and it is a decision that should be, I believe, supported by everybody who recognises that we need to re-establish the worldwide norms in relation to the use and the prohibition of the use of chemical weapons".
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Activists on the ground in Syria said the attack killed more than 40 people and injured hundreds more sheltering from bombing in basements beneath the city.
Douma was the last town held by rebels in the eastern Ghouta region outside Damascus, until they surrendered the day after the alleged gas attack.
Kenneth Ward, US ambassador to the OPCW, raised fears that Syria's ally Russian Federation may have sent soldiers to hide evidence after at least 40 people including children died.
The U.S., United Kingdom and France accuse President Bashar Al-Assad of carrying out the April 7 attack, which killed dozens of people.
"We have always been clear that the government has the right to act quickly in the national interest", May said, calling the military action "not just morally right but also legally right". He denied that Russian Federation was hampering the mission and suggested the approval was held up because of the Western airstrikes. A Pentagon spokeswoman said there was no USA military activity in the area.
The kingdom has supported Sunni rebel groups fighting Assad's forces, which are backed by Shiite-majority Iran.
But, MPs did not get to vote on whether they approved of the decision to attack Syria's chemical weapons facilities. Israel did not confirm or deny mounting the raid.