Syrian authorities — "abetted by Russia's continuing efforts to bury the truth" — still possess and use chemical weapons, an American diplomat told the international chemical weapons watchdog on Thursday.

The strong comments by Kenneth D. Ward, the American ambassador to the Organization for the Prohibition of Chemical Weapons, came amid ongoing diplomatic skirmishes over last week's deadly attack in Syria.

Ward used a hastily convened meeting of the organization's executive council to launch a withering verbal attack on Syrian President Bashar Assad and his allies in Moscow.

The meeting was called to discuss the April 4 attack on the Syrian town of Khan Shaykhun that killed nearly 90 people. The United States and other Western governments blame Assad's regime. Washington in retaliation launched missile strikes on a Syrian air base they say was the starting point for the chemical weapons attack, a move that ratcheted up tensions between the United States and Syria's ally Russia.

Russia and Syria claim the Khan Shaykhun victims were killed by toxic agents released from a rebel chemical arsenal hit by Syrian warplanes.

But Ward insisted it was a deliberate attack that amounted to "a direct affront to the Chemical Weapons Convention and, indeed, a direct affront to human decency, carried out by a State Party" to the OPCW, according to the text of his speech that was posted on the organization's website.

Syria joined the OPCW in 2013 under severe international pressure following a deadly chemical attack on a Damascus suburb. Assad's government told the organization it had a 1,300-ton stockpile of chemical weapons and chemicals used to make them. That stockpile was destroyed in an operation overseen by the Nobel Peace Prize winning-group OPCW, but ever since there have been questions about whether Assad had declared all his weapons.

"On April 4, the lifeless bodies of innocent victims, grotesquely contorted and twisted by the nerve agent sarin, tell the real story," Ward said. "Syria provided a grossly incomplete declaration to the OPCW of its chemical weapons program. Iit continues to possess and use chemical weapons."

He added that "this outrage is abetted by Russia's continuing efforts to bury the truth and protect the Syrian regime" form consequences of using chemical weapons.

Britain's Ambassador, Sir Geoffrey Adams, told the meeting that U.K. scientists have analyzed samples from Khan Shaykhun and they "tested positive for the nerve agent sarin, or a sarin-like substance."

Earlier this week, Turkish doctors also said that test results conducted on victims confirmed that sarin gas was used.

The OPCW's Fact Finding Mission for Syria is conducting an investigation and is expected to report its findings in three weeks. The organization has not revealed any details, citing the need to preserve the integrity of the probe and the safety of OPCW staff.

In Moscow, Russian Foreign Minister Sergey Lavrov said Thursday that OPCW inspectors should visit both the Syrian air base, which the U.S. said served as a platform for the attack, and Khan Sheikhoun to get a full and objective picture.

He said Russia vetoed a draft U.N. resolution Wednesday because it failed to mention the need to inspect the area of the attack.

"We are deeply worried by our partners in the U.N. Security Council trying to evade an honest investigation into that episode," he said.

Lavrov said he emphasized the need for a wide-ranging OPCW probe during Wednesday's talks in Moscow with U.S. Secretary of State Rex Tillerson, suggesting that Western nations, Russia and some regional powers could dispatch additional experts to join the investigation.


Vladimir Isachenkov in Moscow contributed.