* Britain, France cite evidence of sarin gas use in war
* U.N. team wants to know chain of custody of data collected
* Over 80,000 people killed in Syrian civil war -U.N.
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.
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.
Britain said last week that it 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.
Britain's U.N. Ambassador Mark Lyall Grant told reporters on Tuesday that London 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," said Lyall Grant, who is president of the U.N. Security Council for June.
"It is relatively small quantities, but nonetheless, repeated use and any use of chemical weapons is abhorrent," he said.
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.
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.