By Louis Charbonneau
UNITED NATIONS (Reuters) - U.N. investigators reacted cautiously on Wednesday to France's announcement that laboratory tests proved that President Bashar al-Assad's forces had used nerve gas in Syria's civil war, saying it was vital to know the chain of custody of the samples.
French officials said on Tuesday that their tests were the first to comply with international standards and prove that chemical weapons were used in Syria. Paris handed the results to the U.N. chemical weapons investigation team, headed by Swedish scientist Ake Sellstrom.
"Yesterday in Paris, Mr. Sellstrom received additional information related to the reports of the alleged use of chemical weapons in Syria submitted by the government of France," the United Nations said in a statement.
"Mr. Sellstrom cautions that the validity of the information is not ensured in the absence of convincing evidence of the chain-of-custody of the data collected," it said.
French U.N. Ambassador Gerard Araud told reporters on Wednesday that the sources of the samples were reliable.
The Syrian government has denied using chemical weapons and has in turn accused rebels of deploying them in the two-year civil war that the United Nations says has killed over 80,000 people.
The White House also reacted cautiously to the French announcement, saying on Tuesday the United States was not ready to say the Syrian government used chemical weapons in the war-ravaged country.
British U.N. Ambassador Mark Lyall Grant, president of the U.N. Security Council this month, told reporters that Sellstrom's concerns about the samples were understandable. But he said those concerns did not cast doubt on the widespread belief that chemical weapons have been used in Syria.
"I don't think there's anyone who really doubts what we have been saying about the use of chemical weapons in Syria and use by the regime," Lyall Grant said.
"We fully understand that from his (Sellstrom's) point of view there are questions about the chain of custody because he cannot prove where the samples have come from, where they originate and whose hands they passed through," he said.
Lyall Grant said last week that Britain wrote to U.N. Secretary-General Ban Ki-moon about additional suspected chemical weapons attacks in March and April by Syrian government forces. The British letter referred to three different sites in Syria.
Lyall Grant said on Tuesday that London also had evidence of the use of sarin gas and other chemicals in Syria.
"The evidence that we have suggests that there is a use of a number of different variants of chemical agents, a combination of agents in some cases, sometimes including sarin, sometimes not," he said.
"It is relatively small quantities, but nonetheless, repeated use and any use of chemical weapons is abhorrent," he said.
Sellstrom's team of chemical weapons experts has been ready for well over a month to enter Syria to investigate the allegations but has been held up by diplomatic wrangling and safety concerns.
Ban has urged Syria to give the experts unfettered access to investigate all alleged chemical arms incidents. But Assad's government only wants the U.N. team to probe an incident in Aleppo from March. U.N. diplomats say U.N.-Syria negotiations on access have reached a deadlock.
The U.N. statement on Wednesday made clear Sellstrom believes the only way to conduct a proper investigation is by going to Syria.
"On-site activities are essential if the United Nations is to be able to establish the facts," he said.
(Editing by Philip Barbara; Editing by Sandra Maler)