Three UN diplomats confirmed the report's finding.
"The panel is confident that the Syrian Arab Republic is responsible for the release of sarin at Khan Sheikhun on 4 April 2017," the report says, one diplomat told CNN.
The April attack prompted US President Donald Trump to order the US military to launch 59 Tomahawk cruise missiles
at a Syrian airbase.
"Time and again, we see independent confirmation of chemical weapons use by the Assad regime. And in spite of these independent reports, we still see some countries trying to protect the regime. That must end now," Nikki Haley, US ambassador to the United Nations, said in a statement.
Horrifying images and videos emerged after the incident showing civilians
, including children, struggling to breathe with foam coming from their mouths.
Syria denies chemical attack
Syria has repeatedly denied it had anything to do with the attack and also denies it has any chemical weapons. Damascus has said an airstrike hit a chemical weapons depot in the rebel-held area.
The report issued Thursday was compiled by the Organization for the Prohibition of Chemical Weapons (OPCW) and the United Nations' Joint Investigative Mechanism (JIM), the investigative panel probing the use of chemical weapons in Syria. The report was sent to the Security Council.
On Tuesday, Russia vetoed a UN Security Council resolution to extend the mandate of the JIM.
Russia says the group is biased against the Assad government. The mandate of the JIM expires in three weeks.
When the OPCW made a statement about its findings in June,
Syrian ally Russia slammed that news as politically motivated and based on "doubtful data."
Russia is Syria's most powerful ally and has bankrolled much of the six-year conflict, carrying out regular airstrikes in Syria to prop up Assad's regime.
UN Security Council looks to hold Syria accountable
UK Ambassador to the UN Matthew Rycroft told reporters Friday it was time for Russia to "find their moral compass," blasting the country's veto and its government's continued support for the Assad regime.
"Yesterday, the JIM determined responsibility; today, we begin our pursuit of accountability," Rycroft said. "I look to Russia to join in that effort."
"It is long past time for Moscow to abandon Assad," he said. "It is long past time for justice."
Rycroft said that he and his colleagues on the Security Council are discussing a resolution to hold Syira accountable, though he did not provide a timetable for a potential vote on a resolution. It's not yet clear what a resolution would do.
Haley called on the Security Council to "send a clear message that the use of chemical weapons by anyone will not be tolerated."
Stockpiles of sarin may exist in Syria
, despite the OPCW overseeing the destruction of the country's chemical weapon supply in 2013, after an attack in the Ghouta area of Damascus. Activists said that attack killed 1,400 people.
Sarin is an extremely volatile nerve agent because of its ability to change from liquid to gas. People are exposed to sarin through skin contact, eye contact, or by breathing it. Severely exposed people are not likely to survive.
A leading human rights group said the United Nations must act.
"Syria's repeated use of chemical weapons poses a serious threat to the international ban against the use of chemical weapons," said Ole Solvang, the deputy emergencies director at Human Rights Watch, in a statement Friday. "All countries have an interest in sending a strong signal that these atrocities will not be tolerated."
The joint report also blamed ISIS for using sulfur mustard in Syria in September.