"The strike was requested by the partnered forces, who had identified the target location as an Isis fighting position", the Pentagon said. But the countryside south of Raqqa is still under IS control. The Britain-based Syrian Observatory for Human Rights said 25 SDF fighters were killed in the last two days of battle.
The campaign has come at a cost to the predominantly Kurdish forces, which receive ground and air support from the USA military and other allies. "They fabricated the whole story in order to have a pretext for the attack".
The toxic gas attack on April 4, which killed scores of children, prompted the United States to launch missile strikes on a Syrian air base and widened a rift between the United States and Russian Federation, a close Syrian ally.
In Washington, State Department spokesman Mark Toner called Assad's comments "an attempt by him to throw up false flags, create confusion".
Last week's poison gas attack in Khan Sheikhun, Idlib, was the most lethal since a Sarin attack in August 2013 killed hundreds in a rebel-controlled suburb of the capital Damascus.
Matthew Rycroft told the Security Council: "The United Kingdom therefore shares the USA assessment that it is highly likely that the regime was responsible for a sarin attack on Khan Sheikhoun on 4 April". The results, which should arrive in about a month, will be handed to a special United Nations team that will determine who is responsible for using the chemical weapons.More news: United changes policy, crew can't displace seated passengers
This time, the military statement said that the terrorist groups, including IS and the al-Qaida-linked Nusra Front are in possession of chemical weapons.
A chemical attack with a nerve agent did occur, he said, but the available evidence does not support the U.S. government's conclusions.
The tests were not part of an official investigation launched by the Organization for the Prohibition of Chemical Weapons in the wake of the attack.
The council was to vote later Wednesday on a draft resolution demanding the Syrian government cooperate with the investigation, but Russian Federation has said the measure is "unacceptable" and was expected to veto the text. U.S. Ambassador to the U.N. Nikki Haley said the veto left Moscow, a key ally of the Syrian government, with "a lot to prove".
The report offered by the White House, however, cited "a wide body of open-source material" and "social media accounts" from the rebel-held area, including footage provided by the White Helmets rescue group documented to have ties with jihadist rebels, Western and Gulf Arab governments. It is unclear whether they will ever be able to return to their homes. Since the beginning of the conflict, the government has targeted medical workers with detention, torture, and bombardment.