Syrian President Bashar al-Assad denies his government was responsible for a poison gas attack last week in Idlib province that killed dozens of civilians, the news agency Agence France-Presse reported Thursday.

He called the allegation "100 per cent fabrication."

Assad also said Syria's military had given up all chemical weapons, AFP said on its Twitter account, quoting remarks from an interview with the Syrian president.

The United States and its allies say the Syrian military carried out the attack in the town of Khan Sheikhoun, which killed nearly 90 people. The attack prompted a retaliatory missile strike from the U.S. against a Syrian air force base, which further strained relations between Washington, Damascus and its allies in Moscow. 

Assad's remarks come as the U.S. presses its case against Syria before the international chemical weapons watchdog in The Hague. 

Kenneth D. Ward, the U.S. ambassador to the Organization for the Prohibition of Chemical Weapons (OPWC), insisted Syrian forces — "abetted by Russia's continuing efforts to bury the truth" — still possess and use chemical weapons.

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

But Ward, appearing before a hastily convened meeting of the OPWC's executive council, said 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," according to the text of his speech posted on the organization's website.

Britain's Ambassador to The Netherlands Geoffrey Adams, told the meeting that U.K. scientists have analyzed samples from Khan Sheikhoun 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.

MIDEAST-CRISIS/SYRIA-IDLIB

A man breathes through an oxygen mask as another receives treatment, after a gas attack in Khan Sheikhoun, Syria, on April 4. (Ammar Abdullah/Reuters)

Syria joined the OPCW in 2013 under severe international pressure after 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 OPCW, but ever since there have been questions about whether Assad had declared all his weapons.

Separately, the U.S. said Thursday another recent air strike in Syria mistakenly killed 18 of its rebel allies. 

The site of the strike, near the city of Tabqa, had been identified as an ISIS position by unspecified "partnered forces," according to a statement from the Pentagon. The U.S. is leading a coalition of forces against the militant group. 

"The target location was actually a forward Syrian Democratic Forces fighting position," the statement said. 

With files from The Associated Press