Russia accused the U.S., Britain and France on Thursday of inventing several groundless allegations of chemical weapons use in Syria in order to complicate efforts to arrange a U.N. investigation, the latest salvo in the long-running battle over how the world body should handle the country's civil war.

Russia's U.N. Ambassador Vitaly Churkin made the accusation following an announcement that Swedish chemical weapons expert Ake Sellstrom and U.N. disarmament chief Angela Kane will accept a Syrian government invitation to visit Damascus for talks on the terms of a possible investigation.

Churkin said Tuesday that experts from Russia, Syria's closest ally, determined that Syrian rebels made sarin nerve gas and used it in a deadly chemical weapon attack on March 19 in the government-controlled Aleppo suburb of Khan al-Assal, which he said killed 26 people, including 16 military personnel, and injured 86 others.

The rebels have blamed the government for the attack. The U.S., Britain and France reiterated Tuesday that they have seen no evidence to indicate that the opposition has acquired or used chemical weapons.

The Syrian government has called for a U.N. investigation of the Khan al-Assal attack but has refused to expand it to include other alleged chemical weapons attacks in Homs, Damascus and elsewhere raised by the U.S. and its European allies. They have asked the U.N. to investigate at least 10 incidents including Khan al-Assal.

"We need to look into credible allegations," Churkin told reporters Thursday. "Unfortunately, I think what our western colleagues have been doing is trying to produce the maximum number of allegations with minimum credibility in an effort, one might think, to create maximum problems for arranging such investigation."

In June, the United States said it has conclusive evidence that Syrian President Bashar Assad's regime has used chemical weapons against opposition forces, crossing what President Barack Obama has called a "red line" and prompting the U.S. to send arms and ammunition to the opposition, not just humanitarian aid and non-lethal material like armored vests and night goggles.

Acting U.S. Ambassador Rosemary DiCarlo reiterated Thursday that the information provided by the U.S. to the secretary-general was based on "a very thorough assessment by the United States that indicates to a very high degree of confidence that chemical weapons were used on a small scale on multiple occasions."

U.N. spokesman Martin Nesirky indicated that the secretary-general still wants a broader investigation than just Khan al-Assal.

He stressed that guidelines and procedures approved by the U.N. General Assembly state "there can be no substitute for an onsite investigation at all relevant locations in the Syrian Arab Republic."

Nesirky told reporters the secretary-general hopes the visit to Damascus by Sellstrom and Kane will take place as soon as possible so they can arrange for "cooperation for proper, safe and efficient conduct of the mission."

Churkin rejected U.S. State Department charges Wednesday that Russia is working to keep the U.N. from investigating the use of chemical weapons in Syria.

"The notion that Russia is blocking anything is completely misleading," he said, because immediately after the Syrian government called for a U.N. investigation of the Khan al-Assal incident "we have been doing everything we could in order to make sure that that investigation were to happen."

Churkin insisted the rebels were responsible for the Khan al-Assal attack, dismissing a western claim that it was the result of a Syrian government projectile that missed its target. He also said "it makes no sense" that the government would produce the "homemade" projectile and sarin gas used in the March 19 attack.

The Russian ambassador also criticized Britain's attempt to turn a declaration issued at the end of last month's meeting of the Group of Eight major industrialized nations which called for U.N. investigators "to conduct an objective investigation into reports of use of chemical weapons" into a Security Council resolution.

He said the G-8 text, which Russian President Vladimir Putin approved, makes clear that first there should be an investigation and then the results should go to the Security Council for its assessment.

"But you know how our British friends are," Churkin said. "They even drive on the wrong side of the road, so they try to put the cart before the horse once again and we thought that it was completely unreasonable and contrary to the G-8 declaration."

U.S. State Department spokeswoman Jen Psaki questioned why Russia would block a U.N. resolution when it agreed to the G-8 declaration. But she stressed that "the larger issue here is why everybody wouldn't support unfettered access for the U.N. team to investigate chemical weapons use in Syria."