UNITED NATIONS -- The United States and Russia have circulated rival U.N. resolutions on extending the work of experts seeking to determine who is responsible for chemical weapons attacks in Syria.

Russia vetoed a U.S.-sponsored Security Council resolution on Oct. 24 that would have renewed the mandate of the experts from the U.N. and the international chemical weapons watchdog for a year.

Two days after Russia's veto, the Joint Investigative Mechanism (JIM) released a report blaming Syria's government for a sarin nerve gas attack last April on the town of Khan Sheikhoun that killed more than 90 people and the Islamic State of Iraq and Syria (ISIS) extremist group for a mustard gas attack at Um Hosh in Aleppo in September 2016.

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The inspectors of the JIM, who were unable to visit Khan Shaykun, based their findings on "sufficient credible and reliable evidence" of Syrian aircraft that dropped munitions, the crater that was caused by the impact of the aerial bombing, interviews with victims, and their finding that sarin identified in the samples taken from Khan Shaykhun was most likely been made with a precursor (DF) from the original stockpile of the Syrian Arab Republic, CBS News' Pamela Falk reports.

Unless a new agreement is struck to extend the mandate of the inspectors before mid-November, Falk reports, Russia's October veto will prevent inspectors from doing their job and investigating chemical weapons attacks in Syria, but it is likely that, with duelling resolutions, there will be a compromise resolution agreed to before the deadline, a Security Council diplomat told CBS News, although U.S.-Russian acrimony on this topic as reached a "fevered pitch."

Both the Russian draft resolution and the U.S. version obtained on Thursday by CBS News would renew the mandate of the JIM, but under very different conditions.

The White House issued a statement Thursday blasting Syrian President Bashar Assad's regime.

"This unconscionable attack marks the fourth time that the JIM has confirmed that the Assad regime used chemical weapons, underscoring the brutal and horrifying barbarism of Bashar al-Assad and making the protection provided by Russia even more egregious."

Russia, a close ally of Syria, has strongly criticized the JIM, finding fault with its methods of collecting evidence and its failure to visit the sites of alleged attacks. In vetoing the resolution, Russia's U.N. ambassador, Vassily Nebenzia, said Moscow wanted to wait for the JIM report.

Last year, the JIM determined that Assad's government was behind at least three attacks involving chlorine gas, a finding that Russia disputed.

The U.S. draft resolution circulated to the Security Council expresses "grave concern" at the JIM's reports, especially its latest findings on Khan Sheikhoun and Um Hosh.

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 for the attack and launched a punitive strike days later on the Shayrat air base where it said the attack was launched. Syria has denied using chemical weapons, and Russia has strongly backed Assad's government.

A fact-finding mission by the chemical weapons watchdog, the Organization for the Prohibition of Chemical Weapons, reported on June 30 that sarin nerve gas was used in the Khan Sheikhoun attack.

But the job of determining responsibility for chemical attacks was given to the JIM, initially in a joint U.S.-Russian resolution.

The U.S. draft resolution would reaffirm support to the OPCW fact-finding mission and the JIM "as they undertake their respective investigations in a manner they deem appropriate to fulfill their mandate." It would renew the JIM's mandate, which expires on Nov. 17, for two years.

Russia's draft would extend the JIM's mandate for six months, until May 16, 2018. It states clearly that "there is the need for further improvements and update of the JIM's mandate."

In Moscow, Mikhail Ulyanov, head of the Russian Foreign Ministry's arms control and non-proliferation department, harshly criticized the latest JIM report and said the ground rules must be changed to include on-site inspections.

He argued that the report on Khan Sheikhoun was unsubstantiated and ignored evidence suggesting that sarin could have been used by Syrian rebels in order to blame Assad's government.

Ulyanov particularly criticized the JIM for failing to take samples from the site of the attack in Khan Sheikhoun and the Shayrat air base despite security guarantees offered to inspectors.

"Imagine a criminal investigation in which police refuse to visit the site of the crime. No court will ever accept it," Ulyanov said. "But they consider it possible to do such thing at the U.N. Security Council."

He said the JIM's conclusion that a Syrian warplane dropped a bomb containing sarin isn't supported by images of the explosion site that show a crater that he said could only have been left by an explosive device planted on the surface.

Ulyanov also said photos and videos made by the White Helmets volunteer first responders at the Khan Sheikhoun attack site show them without proper protection gear -- an indication he argued that the attack was a fabrication.

The Russian draft resolution urges the JIM to send investigators to Khan Sheikhoun and Shayrat as soon as possible -- and it urges the JIM "to reevaluate its earlier assessments and conclusions."

If on-site investigations can't be organized to investigate a chemical incident, the draft urges the OPCW to inform JIM so the Security Council can be made aware of the problem.

The proposed resolution would also order the JIM to conduct its investigations "guided by high standards" established by the Chemical Weapons Convention.

It asks the JIM to collect and analyze information and evidence that is not from the OPCW fact-finding mission, including material from U.N. counterterrorism and nonproliferation bodies and the Syrian government.

The draft also asks Secretary-General Antonio Guterres to ensure that JIM staffers are recruited from "as wide a geographical area as possible."

The Security Council is scheduled to discuss the latest JIM report on Tuesday.