UNITED NATIONS (AP) — Experts from the U.N. and the chemical weapons watchdog are blaming Syria's government for a sarin nerve gas attack that killed over 90 people last April.

Their report, obtained Thursday by The Associated Press, says leaders of the expert body are "confident that the Syrian Arab Republic is responsible for the release of sarin at Khan Sheikhoun on April 4, 2017."

The report supports the initial findings by the United States, France and Britain that a Syrian military plane dropped a bomb with sarin on the town.

Syria and Russia, its close ally, have denied any attack and have strongly criticized the Joint Investigative Mechanism, known as the JIM, which was established by the U.N. and the Organization for the Prohibition of Chemical Weapons to determine responsibility for chemical weapons attacks in Syria.

The attack in Khan Sheikhoun sparked outrage around the world as photos and video of the aftermath, including quivering children dying on camera, were widely broadcast.

The United States blamed the Syrian military and launched a punitive strike days later on the Shayrat air base, where it said the attack was launched.

Responding to the report, U.S. Ambassador Nikki Haley said: "Today's report confirms what we have long known to be true. Time and again, we see independent confirmation of chemical weapons use by the Assad regime."

Clearly referring to Russia, she said: "In spite of these independent reports, we still see some countries trying to protect the regime. That must end now."

The Security Council should make it clear that "the use of chemical weapons by anyone will not be tolerated," Haley said.

A fact-finding mission by the Organization for the Prohibition of Chemical Weapons reported June 30 that sarin was used in the Khan Sheikhoun attack and "sulfur mustard" in Um Hosh. But the JIM experts had the task of determining who conducted the attacks.

The JIM experts said Thursday they are "confident" the Islamic State extremist group was responsible for an attack in Um Hosh in Aleppo on Sept. 15-16, 2016, that used "sulfur mustard," the chemical weapon commonly known as mustard gas.

In addition to blaming the Syrian government and the IS group, the JIM report says, "The continuing use of chemical weapons, including by non-state actors, is deeply disturbing."

"If such use, in spite of the prohibition by the international community, is not stopped now, a lack of consequences will surely encourage others to follow — not only in the Syrian Arab Republic but also elsewhere," it warns. "This is the time to bring these acts to an end."

The report was issued two days after Russia vetoed a U.S.-sponsored resolution to extend the mandate of the JIM investigators for another year after it expires Nov. 17.

Russian Ambassador Vassily Nebenzia said Moscow wanted to wait for the JIM report. "We can meaningfully negotiate the renewal of JIM after we have seen the report," he told reporters Thursday before it came out.

Russia's U.N. Mission said after its circulation: "We have started a thorough study of this paper, which is of very complex technical nature."

Haley said countries that don't support the JIM experts "are no better than the dictators or terrorists who use these terrible weapons."

The investigators determined last year that the Syrian government was behind at least three attacks involving chlorine gas and that the Islamic State extremist group was responsible for at least one involving mustard gas.

Louis Charbonneau, U.N. director for Human Rights Watch, said: "The question now is whether Security Council and OPCW members, including Russia, will move to protect a key international rule and hold Syrian authorities accountable as they said they would."

British Foreign Secretary Boris Johnson said: "Britain condemns this appalling breach of the rules of war and calls on the international community to unite to hold Assad's regime accountable."

The report said the JIM experts talked with 17 witnesses in addition to those interviewed by the OPCW fact-finding mission and collected and reviewed material the OPCW did not have. It said the experts also obtained "substantial information" on activities by the Syrian air force on April 4.

The experts determined sarin was released from a crater in the northern part of Khan Sheikhoun between 6:30 a.m. and 7 a.m. April 4.

Based on photos, videos and satellite images as well as studies of munition remnants by forensic institutes and individual experts hired by the JIM, the experts "assessed that the crater was most probably caused by a heavy object traveling at a high velocity, such as an aerial bomb with a small explosive charge," the report said.

The possibility that an improvised explosive device caused the crater "could not be completely ruled out," but the experts determined that was "less likely" because an IED "would have caused more damage to the surroundings than had been observed at the scene."

The report said the investigators received information that Syrian air force planes "may have been in a position to launch aerial bombs in the vicinity" of the town. But it said air force flight records and other records provided by Syria's government made no mention of Khan Sheikhoun. The experts said they received "conflicting information" about aircraft deployments in the town that morning.

The experts also obtained "original video footage from two separate witnesses that showed four plumes caused by explosives across Khan Sheikhoun," the report said. It said forensic analysis confirmed the footage was made during the time of the sarin attack.

According to the report, the JIM leadership panel concluded the Syrian military was behind the sarin attack based on the following "sufficient, credible and reliable evidence":

—Aircraft dropped munitions over Khan Sheikhoun between 6:30 a.m. and 7 a.m. on April 4

—Syrian aircraft were "in the immediate vicinity" at that time.

—The crater was created that morning.

—The crater "was caused by the impact of an aerial bomb traveling at high velocity."

—The number of people affected and the presence of sarin at the crater 10 days later "indicate that a large amount of sarin was likely released, which is consistent with it being dispersed via a chemical aerial bomb"

—The symptoms of victims, their treatment and the scale of the incident "are consistent with a large-scale intoxication of sarin."

—Sarin samples from Khan Sheikhoun were "most likely" made with a precursor chemical that was from "the original stockpile of the Syrian Arab Republic."

U.N. Secretary-General Antonio Guterres reiterates "his full confidence in the professionalism, impartiality and objectivity" of the JIM and looks forward to the Security Council's consideration of the report Nov. 7, U.N. deputy spokesman Farhan Haq said.