Victims of a suspected chemical weapons attack in Khan Sheikhoun, Syria, on April 4. (Alaa Alyousef via AP)

The nerve agent sarin was used in a chemical attack in April that killed scores of civilians in northern Syria, a global watchdog confirmed Friday, days after the White House accused President Bashar al-Assad’s government of planning a similar strike.

In a statement released ahead of a more thorough fact-finding report, the Organization for the Prohibition of Chemical Weapons (OPCW) described the daybreak assault on the opposition-held town of Khan Sheikhoun as an “atrocity.”

The OPCW’s findings first leaked Thursday when Nikki Haley, the U.S. ambassador to the United Nations, said that the watchdog had confirmed the use of sarin, heightening growing concerns over the Assad government’s chemical stockpile, three years after it was supposed to have been destroyed under the scrutiny of international inspectors.

Entire families were killed in the April 4 attack, some while they were sleeping. A nearby hospital was bombed hours later, forcing rescue workers to load convulsing casualties into pickup trucks and drive them to the next nearest clinic, a half-hour away.

The OPCW investigation did not have a mandate to apportion blame, referring the case instead to a United Nations investigative body. But in Brussels, British Defense Secretary Michael Fallon said his government’s own assessment indicated that Assad’s military was responsible.

“We have our own evidence that it’s highly likely the regime did it, so we expect that report to confirm that it did. Clearly the gas was dropped from the air,” he said, in reference to the capabilities of the Syrian air force. 

The OPCW conclusions, due to be published in full July 5, are based on a wide range of gathered evidence, including the results of analysis of victims and environmental samples, and extensive witness interviews.

The Syrian government denies that it has chemical weapons, and Assad’s supporters have attributed the deaths in Khan Sheikhoun to the accidental bombing of a jihadist-run chemical weapons factory.

Assad’s military was supposed to have surrendered its chemical stockpiles to international inspectors in 2014. But Western diplomats and the inspectors themselves had long suspected that a portion of the stockpile was never declared.

The OPCW said in a statement last month that its fact-finding team was working to clarify “unresolved issues” over the Syrian government’s declared chemical stockpiles.

Images of Khan Sheikhoun’s casualties, many of them children, prompted President Trump to order missile strikes on the air base from which the Syrian warplanes had flown.

This week, U.S. officials said they had observed indications that the base was being used to prepare fresh chemical attacks. In a statement late Monday, the White House warned that Assad would pay a “heavy price” for any such strike.

Chemical weapons experts say the Syrian government has used its supplies of toxic agents primarily to depopulate civilian areas and strike fear into those who remain there.

U.S. officials have monitored leadership figures and researchers from a Syrian chemical warfare unit moving between facilities linked to the production of weapons in recent weeks, according to an intelligence analyst who spoke on the condition of anonymity because they were not authorized to speak with the media.

The revelations indicated that Washington has monitored Syria’s chemical weapons program more closely than was previously known.

On the morning of the April 4 attack, a network of civilian observers issued an alert as Syrian warplanes took off from the nearby Shayrat air base and headed toward Khan Sheikhoun. 

As the aircraft circled in the sky, an observer radioed colleagues to warn of an imminent attack. “Guys, tell people to wear masks,” the observer said, according to a transcript. “It has chemicals with it. I am sure of that.”

Thomas Gibbons-Neff in Brussels contributed to this report.