WASHINGTON/UNITED NATIONS (REUTERS) - The United States on Friday (April 13) blamed the Syrian government for a deadly chemical weapons attack this month and slammed Russia for failing to stop its ally, President Bashar al-Assad, as Western allies considered military strikes on Syria.

While the prospect of US-led military action that could lead to confrontation with Russia hung over the Middle East, the White House accused Syria of carrying out a toxic gas assault on April 7 that killed dozens of people in Douma, near Damascus.

"We have a very high confidence that Syria was responsible and, once again, Russia's failure to stop them and their continued (lack of action) on this front has been part of the problem," White House spokeswoman Sarah Sanders said.

US intelligence shows a Russian claim that the attack was faked was false, Sanders said.

"Our intelligence tells us otherwise. I can't go beyond that," Sanders told reporters.

The US State Department said the United States has proof at "a very high level of confidence" that the Syrian government carried out the attack but is still working to identify the mix of chemicals used.

President Donald Trump warned on Wednesday that missiles "will be coming" in response to the Douma incident. Russia has told the United States and its allies not to carry out any military strike.

Chemical weapons experts for the Organisation for the Prohibition of Chemical Weapons (OPCW) arrived in Syria to investigate the suspected poison gas attack.

The investigators, who are only mandated to determine if chemical weapons were used and not who used them, were expected to start their investigations into the Douma incident on Saturday, the Netherlands-based organisation said.

Asked whether the United States was waiting for the OPCW's report before it makes a decision on Syria, State Department spokeswoman Heather Nauert said, "We believe we know who is responsible for this. We know a chemical weapon was used."

"The OPCW will still formulate its facts but the OPCW does not determine the responsibility. They just determine the substance," Nauert added.

RUSSIA WARNING

Russian Foreign Minister Sergei Lavrov said earlier that Moscow was in contact with Washington to discuss an atmosphere that he described as alarming.

"God forbid anything adventurous will be done in Syria along the lines of the Libyan and Iraqi experience," Lavrov told a news conference, referring to past Western military interventions elsewhere in the region.

Britain's UN Ambassador Karen Pierce, meanwhile, rejected a charge by a Russian defense ministry spokesman that Britain was involved in staging a fake chemical weapons attack in Douma.

"This is grotesque, it is a blatant lie, it is the worst piece of fake news we've yet seen from the Russian propaganda machine," Pierce told reporters.

Assad is supported by Iranian-back fighters as well as the Russian air force and has cemented his control over most of the western, more heavily populated, part of the country.

The capture of Douma from rebels who evacuated this week has clinched a major victory for Assad, crushing what was once a center of the insurgency near Damascus, and underlines his seemingly unassailable position in the war.

While Trump himself was silent on Syria on Friday, giving no further clues on whether American military action is imminent, US Ambassador to the United Nations Nikki Haley said Washington estimated that Assad's forces had used chemical weapons at least 50 times during the seven-year-old Syrian conflict.

"Our president has not yet made a decision about possible action in Syria. But should the United States and our allies decide to act in Syria, it will be in defense of a principle on which we all agree," Haley told the UN Security Council.

"All nations and all people will be harmed if we allow Assad to normalise the use of chemical weapons."