A visit by chemical weapons inspectors to the site of a suspected gas attack in Syria was delayed on Monday, British and Russian officials said, as Western powers and Russia traded accusations in the aftermath of retaliatory U.S.-led missile strikes.

Moscow, the main ally of Syrian President Bashar al-Assad, on Sunday condemned the United States, Britain and France for refusing to wait for the findings of the Organization for the Prohibition of Chemical Weapons (OPCW) inspection team on the alleged attack on Douma before they launched the strikes.

OPCW inspectors arrived in Damascus on Saturday and had planned to head to Douma, on the outskirts of the capital, on Monday.

But the British delegation to the OPCW said Russia and Syria had not yet allowed inspectors access to Douma.

Britain's Ambassador Peter Wilson said at a news conference 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.

"Unfettered access [is] essential," a British statement said. "Russia and Syria must co-operate."

Russia's deputy foreign minister said the delay was due to the Western strikes.

But later Monday, speaking from The Hague, Russian Defence Ministry official Igor Kirillov said that experts from the OPCW will travel to Douma on Wednesday.

It is long overdue that this council condemns the Syrian government for its reign of chemical terror - Kenneth Ward, U.S. ambassador

The U.S. envoy to the global watchdog said Russia may have tampered with the site of the April 7 attack, which aid organizations say killed dozens of men, women and children.

U.S. Ambassador Kenneth Ward levied the accusation against Russia during a closed-door meeting at the OPCW, but Russian Foreign Minister Sergei Lavrov has denied allegations of interference.

"It is long overdue that this council condemns the Syrian government for its reign of chemical terror and demands international accountability for those responsible for these heinous acts," Ward said in comments obtained by Reuters.

"It is our understanding the Russians may have visited the attack site. 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."

The United States, Britain and France fired more than 100 missiles at three alleged chemical weapons facilities early Saturday, angering Syria's military backer, Moscow, which threatened to retaliate.

U.S. President Donald Trump said the strikes had accomplished their aim of undermining efforts by the Syrian government to produce and use chemical weapons again in violation of the Chemical Weapons Convention.

OPCW inspectors were trying to gain access to the sites in Douma on Monday aiming to collect samples, interview witnesses and document evidence to determine whether banned toxic munitions were used.

Evidence may have been removed

It has been more than a week since the attack in which witnesses and Western governments described helicopters dropping sarin and chlorine bombs that killed many children and women hiding from clashes between rebels and government troops.

A diplomatic source told Reuters that evidence may have been removed while inspectors negotiated access with Syrian authorities.

But on Monday, Lavrov told BBC: "I can guarantee that Russia has not tampered with the site."

Increasing regional jitters, Syrian anti-aircraft defences shot down missiles fired at the Syrian air base of Shayrat in Homs province, Syrian state television said.

The Pentagon said there was no U.S. military activity in that area "at this time." Separately, the pro-Iranian Hezbollah militia said Syrian air defences intercepted three missiles aimed at Dumair military airport northeast of Damascus.

An Israeli military spokesperson said, "We don't comment on such reports."

Syria and Russia deny chemical weapons were used in the final offensive that captured Douma, a rebel-held territory east of Damascus.

(CBC)

The British envoy to the OPCW said it had recorded 390 allegations of the use of banned chemicals in Syria since 2014, and that a failure by the OPCW to act risked allowing "further barbaric use of chemical weapons."

Syria joined the OPCW, the organization tasked with monitoring adherence to the 1997 convention, in 2013 after a sarin gas attack that killed hundreds of people in Ghouta. The move was part of a joint Russian-U.S. deal that averted military action threatened by then-president Barack Obama.

Putin warns against further attacks

Russian President Vladimir Putin warned on Sunday that further Western attacks on Syria would bring chaos to world affairs, as Washington prepared to increase pressure on Russia with new economic sanctions.

Members of the 41-seat executive council of the OPCW were due to discuss the alleged use of prohibited toxins in Syria, but were not expected to reach any agreement about a response.

The organization, which needs a two-thirds majority to make decisions, has been undermined by deep political division over the use of chemical weapons by the Syrian government. The OPCW inspectors will not assign blame for attacks.

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. - Boris Johnson, British foreign secretary

A joint United Nations-OPCW mission concluded that troops under Syrian President Bashar al-Assad had used chemical weapons several times in recent years, including in a sarin attack a year ago in the town of Khan Sheikhoun that killed nearly 100 people.

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."

Rubble lines a street in Douma, Monday, the site of the suspected chemical weapons attack. (Hassan Ammar/Associated Press)

NATO Secretary General Jens Stoltenberg has defended the weekend's airstrikes in Syria, saying the international community had to uphold the international ban on the use of chemical weapons.

Stoltenberg spoke at a joint news conference with Turkey's foreign minister on Monday, while in the country for talks with President Recep Tayyip Erdogan and other officials.

He said Russia had "again and again" prevented an independent investigation by the UN and that this left "NATO allies no other alternative than to act the way they acted."

The Western alliance "cannot be silent where chemical weapons are used," he said. "There was more than enough reason to act, and not to act would be to erode the ban on chemical weapons."

More U.S. sanctions

Nikki Haley, the U.S. ambassador to the United Nations, said Sunday the United States would announce new economic sanctions aimed at companies dealing with equipment related to Assad's alleged use of chemical weapons.

Responding to Haley's remarks, Evgeny Serebrennikov, deputy head of a Russian parliamentary defence committee, said Moscow was ready for the penalties.

"They are hard for us, but will do more damage to the USA and Europe," RIA news agency quoted Serebrennikov as saying.

Britain's decision to conduct airstrikes against Syria was in the country's national interest, and not the result of pressure from Trump, Prime Minister Theresa May told Parliament on Monday.

"We have not done this because President Trump asked us to; we have done it because we believed it was the right thing to do, and we are not alone," May said. "There is broad-based international support for the action we have taken.

"Nobody should be in doubt of our resolve — that we cannot see a situation when the use of chemical weapons is normalized."