(Adds British ambassador, paragraphs 9-10)
By Louis Charbonneau and Michelle Nichols
UNITED NATIONS, Dec 16 (Reuters) - Russia on Monday accused Washington of ignoring Moscow's appeals for proof of Syrian government involvement in chemical weapons attacks in Syria during the country's more than 2-1/2-year-old civil war.
Moscow's allegations against Washington come as both say they are encouraging peace talks that Western powers and Gulf Arab states hope will lead to the departure of Syrian President Bashar al-Assad and the formation of a transitional government.
"Our requests for additional information which could prove the Syrian government involvement in the use of chemical weapons were ignored by Washington," Russian U.N. Ambassador Vitaly Churkin told reporters after a closed-door meeting on chief U.N. chemical arms investigator Ake Sellstrom's final report.
Churkin said allegations about Syrian government involvement in chemical attacks, including an Aug. 21 sarin gas attack that killed hundreds of people, "were not persuasive." He said Moscow viewed the Aug. 21 incident as a provocation by the rebels.
The Russian ambassador referred to what he said was information released by former U.S. spy agency contractor Edward Snowden. U.S. relations with Russia have been strained in recent months over Moscow's decision to grant temporary asylum to Snowden, who leaked documents about widespread U.S. surveillance activity.
"The lack of any proof (about Syrian government use of chemical arms) was particularly strange because as the public learned from the media reports based on Edward Snowden's materials the United States had powerful intelligence assets in Syria," Churkin said.
He described those intelligence assets as "sensors capable to provide in real time on any mixing of precursors by the Syrian troop for sarin production. Sarin does not live long."
Sellstrom's inquiry was only looking at whether chemical weapons were used, not who used them. The Syrian government and the opposition have accused each other of using chemical weapons, and both have denied it.
British Ambassador Mark Lyall Grant told reporters that "Russia was using the classic diplomatic giant squid defense - if you squirt enough ink into the water, you try to muddy the waters so that people aren't clear about what happened."
He added that all of Sellstrom's reporting, both written and oral, has left no doubt that the Syrian government was responsible for multiple chemical attacks on its own people.
The Aug. 21 sarin gas attack, which Washington said killed more than 1,400 people, many of them children, led U.S. President Barack Obama to threaten air strikes against Syrian military facilities because it had crossed Obama's "red line" for intervening. Facing the threat of U.S. attacks, the Syrian government agreed to dismantle its chemical arms program and describe its arsenal.
"Why would the Syrian government use chemical weapons on Aug. 21?" Churkin said. "To cross the red line drawn by Washington and invite a missile strike upon itself?"
"Why would the opposition use chemical weapons? Exactly because of the red line," he said. "To provoke foreign military intervention in the Syrian conflict. It is absolutely obvious that on Aug. 21 a large-scale provocation was staged."
The U.S. mission did not react immediately to Churkin's remarks, though diplomats said U.S. Ambassador Samantha Power responded to him behind closed doors. She told Churkin a "Christmas vacation might do the Russian ambassador good," according to a diplomat present.
Churkin told Reuters it "was not a very polite comment to make."
Power also told the 15-nation council that the "Russian regime has a remarkable trust in a government that sends rockets at and bombs its own population," the diplomat present at the meeting added on condition of anonymity.
Churkin told reporters Russia does not trust Syria but has looked at the facts to come up with its assessments.
Sellstrom's report, released on Thursday, said poison gas was likely used in five out of seven incidents investigated by U.N. experts in Syria, where the civil war has killed more than 100,000 people. It said sarin was probably used in four attacks.
The opposition Syrian National Coalition said Sellstrom's report implicated Assad and showed that "urgent action, including a referral of the situation in Syria to the International Criminal Court, must be taken to ensure that those responsible for atrocities are held to account."
Diplomats said that one of the most telling details in Sellstrom's report is the fact that helicopters may have been used in some of the attacks. If helicopters were involved, diplomats say, it would implicate the government, since the rebels do not have helicopters.
The report says that there were "military helicopters or airplane overflights" at the time of what Washington said was a chemical attack in Aleppo on April 13. It also referred to reports that a likely gas attack in Saraqeb on April 29 may have involved helicopter activity.
Earlier on Monday, U.N. Secretary-General Ban Ki-moon said the fighting in Syria must cease before political negotiations on a transitional government can begin.
"We must have a cessation of hostilities before we begin political dialogue on Syria in Geneva," Ban told reporters in New York. "This fighting must stop."
Representatives of Assad's government and the opposition are scheduled to meet with peace mediator Lakhdar Brahimi in Switzerland on Jan. 22 to discuss ways of ending the war.
Churkin did not respond to a question about whether Russia, which is a strong ally of Assad, would support a ceasefire. (Editing by Andrew Hay and Mohammad Zargham)