UNITED NATIONS - The United Nations on Wednesday defended a report by UN chemical weapons experts that Russia has criticized as "one-sided," saying its conclusion that rockets loaded with sarin gas were used in an August 21 attack should not be questioned."The findings in that report are indisputable," UN spokesman Martin Nesirky told reporters. "They speak for themselves and this was a thoroughly objective report on that specific incident."UN chemical investigators led by Ake Sellstrom of Sweden on Monday confirmed the use of sarin nerve agent in the August 21 attack outside the Syrian capital in a long-awaited report that the United States, Britain and France said proved President Bashar Assad's forces were responsible.Russia, however, denounced Sellstrom's findings as preconceived and tainted by politics, stepping up its criticism of the report. Russia, like Assad's government, says the rebels carried out the attack, which the United States says killed more than 1,400 people, including over 400 children.Nesirky said the chain of custody of all environmental and biomedical samples taken by the inspectors was meticulously documented. He added that the experts would return to Syria as soon as possible to continue their investigation into a March incident at Khan al-Asal and all other "credible allegations."Sellstrom's mandate was limited to investigating whether chemical weapons were used, not who used them. But Western officials say that technical details in the report provide clear evidence that Assad's forces carried out the attack.The advocacy group Human Rights Watch said on Tuesday that rocket trajectories detailed in the UN report suggested the sarin-filled shells had been fired from a base belonging to the Republican Guard, run by Assad's brother, Maher.Western diplomats confirmed the Human Rights Watch report. Diplomats in New York said Russian UN Ambassador Vitaly Churkin questioned some of the findings in Sellstrom's report at a Security Council meeting on Monday. Churkin, they said, asked Sellstrom to describe the quality of the weapons that dispersed sarin."The rockets found on the site were professionally made and, according to Dr. Sellstrom, they bore none of the characteristics of jerry-rigged, improvised weapons," US Ambassador Samantha Power said on Tuesday."They had sophisticated barometric fuses to disperse the nerve agent in the air and not on impact," she said. "This was a professionally executed massacre by the regime, which is known to possess one of the world's largest undeclared stockpiles of sarin."Separately, diplomats from the United States, Russia, Britain, France and China were hoping to continue negotiations on Wednesday on a Western-drafted resolution that would demand the destruction of Syria's chemical arsenal in line with a US-Russian deal agreed last weekend.But one senior Western diplomat said they were waiting to hear from Russia and China about whether they received instructions from Moscow and Beijing overnight before setting up a new meeting on Wednesday.