An unconscious child receives treatment at a hospital in Khan Sheikhoun, a town in northwest Syria, on April 4, 2017, after a suspected chemical attack. (Omar Haj Kadour/AFP/Getty Images)

Russia used its veto power on the United Nations Security Council on Tuesday to block an extension of efforts by international inspectors to determine who was behind chemical weapons attacks that have killed scores of Syrian civilians.

Moscow's veto decision was condemned by the United States, Britain and others as an attempt to shield the perpetrators from answering for the most controversial human rights abuses of Syria's six-year-old war.

Western intelligence officials and U.N. investigators have blamed the government of Syrian President Bashar al-Assad for the attacks.

It was the ninth time Russia has used its veto to hinder international action on Syria. Moscow is a key ally of the Syrian government, supporting it militarily, politically and financially.

[U.N. accuses Syrian government of April sarin attack]

A U.N. investigative body is due to release a report Thursday attributing blame for an April 4 chemical attack on the northern town of Khan Sheikhoun.

More than 80 people were killed, and hundreds were injured in the daybreak assault, in which the Syrian government is alleged to have used the banned nerve agent sarin.

Russia's U.N. ambassador, Vassily Nebenzia, had tried unsuccessfully to have Tuesday's Security Council vote postponed until the contents of the report were released.

President Trump ordered retaliatory missile strikes days after the Khan Sheikhoun assault, targeting the Syrian government air base from which the warplanes that carried out the chemical attack were thought to have departed.

[Analysis: A chemical weapons attack in Syria exposes Trump’s Assad problem]

A separate U.N. inquiry also has accused Syria of responsibility for the Khan Sheikhoun assault, saying it was one of more than 20 government attacks involving chemical weapons since March 2013, most of them targeting families with no role in the conflict. This investigation, formally known as the U.N. Commission of Inquiry on Syria, is tasked with probing war-crimes allegations and has no mandate to prosecute any party.

The mandate of the ongoing U.N.-backed inquiry into chemical weapons attacks in Syria, known as the Joint Investigative Mechanism, or JIM, expires Nov. 17. It also has several other attacks to investigate.

The U.S.-drafted resolution put to a Security Council vote on Tuesday would have extended the JIM's mandate, raising the prospect of accountability for a chapter of human rights abuses that has drawn unparalleled international condemnation.

Amnesty International described the Russian veto as the equivalent of "a green light for war crimes."

"By preventing the mandate extension of the gas attacks probe they helped set up, Russia has dealt a huge blow [to] justice in Syria and shown once again their callous disregard for all those who have been killed and injured in these attacks," said Sherine Tadros, head of Amnesty International's U.N. office in New York.

[Chemical weapons activity monitored at Syrian air base, U.S. officials say]

It was unclear Tuesday whether the United States and allies on the Security Council would try for a second vote to continue the JIM's work.

"It is not every day that this council considers an issue that is so horrific, so shocking to the conscience as the use of chemical weapons against civilians," said Michele J. Sison, the deputy U.S. representative to the United Nations, addressing the chamber. "We want to know the truth about these attacks regardless of where it takes us."

Correction: A previous version of this story misspelled Michele J. Sison's name.

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