* Projectile carrying sarin likely built by rebels-Russia
* Government, rebels accuse each other of chemical attacks
Russian scientific analysis indicates a deadly projectile that hit a suburb of the Syrian city of Aleppo on March 19 contained the nerve agent sarin and was most likely fired by rebels, Russia's U.N. envoy said on Tuesday.
The incident at Khan al-Assal in the northern province of Aleppo killed more than two dozen people. Both the government and rebels have blamed each other for what they say was an attack involving chemical weapons. Both sides also deny using chemical weapons.
Russia, alongside Iran, is Syria's closest ally and chief arms supplier. The United States cast doubt on the Russian analysis and along with France called for full U.N. access to Syrian sites where chemical weapons use was suspected.
Moscow's U.N. ambassador, Vitaly Churkin, said Russian experts visited the location where the projectile struck and took their own samples of material from the site. Those samples, he said, were then analyzed at a Russian laboratory certified by the Organization for the Prohibition of Chemical Weapons.
"It was established that on March 19 the rebels launched an unguided Basha'ir-3 projectile towards Khan al-Assal controlled by the government forces," he said. "The results of the analysis clearly indicate that the ordnance used in Khan al-Assal was not industrially manufactured and was filled with sarin."
"The projectile involved is not a standard one for chemical use," Churkin said. "Hexogen, utilized as an opening charge, is not utilized in standard ammunitions. Therefore, there is every reason to believe that it was armed opposition fighters who used the chemical weapons in Khan al-Assal."
Churkin said he had informed U.N. Secretary-General Ban Ki-moon of the Russian findings. Ban is scheduled to meet Ake Sellstrom, the Swedish scientist heading a U.N. team established to investigate allegations of chemical weapon use in Syria, in New York this week.
White House spokesman Jay Carney said the United States had not seen any evidence to suggest anyone other than the Syrian government "had the ability to use chemical weapons, or has used chemical weapons."
"If (Syrian President) Bashar al-Assad is seriously interested in proving his assertion, and now the assertion that Russia is making, Assad should let the U.N. investigators in and Russia should use its relationship with Assad to press Assad to allow U.N. investigators in," Carney said.
So far, Sellstrom's team has not traveled to Syria. Ban wants Sellstrom to have unfettered access to investigate all alleged chemical attacks while Assad's government wants the U.N. experts to confine their investigation to the March 19 incident.
"We need to have a full-fledged investigation by the U.N. into all the allegations in Syria," said French U.N. Ambassador Gerard Araud. "Sellstrom has to go to all the sites of the allegations to investigate."
Churkin said the projectile used to deliver sarin on March 19 was constructed by rebels fighting to oust Assad. The United Nations estimates that over 90,000 people have died in Syria's two-year civil war.
"According to the information at our disposal, the production of Basha'ir-3 unguided projectiles was started in February 2013 by the so-called 'Basha'ir al-Nasr' brigade affiliated with the Free Syrian Army," he said.
Additional details about the Basha'ir-3 were not available.
Ban spokesman Martin Nesirky was asked about Churkin's statement on Tuesday. He confirmed receipt of the Russian report on the Khan al-Assal incident, adding that Ban "takes seriously all credible allegations."
Syrian U.N. Ambassador Bashar Ja'afari on Monday said his government has invited Sellstrom and U.N. disarmament chief Angela Kane to Damascus to discuss allegations of banned arms use in Syria's two-year civil war but suggested it would not compromise on access.
One senior Western diplomat said last month that Britain and the United States alone have notified Ban of 10 separate incidents of the use of chemical weapons by Assad's forces. France has also said its own tests of samples from inside Syria proved Assad's forces have used the nerve agent sarin.
Sellstrom is expected to deliver an interim report this month. U.N. envoys say it likely will be inconclusive since he cannot be certain about the chain of custody of the samples he has received from Britain, France and the United States.
Churkin said the Russian experts took their own samples and did not rely on third parties.
Syria is one of seven countries that has not joined the 1997 convention banning chemical weapons. Western nations believe it has caches of undeclared mustard gas, sarin and VX nerve agents.