They spoke on condition of anonymity because the letter has not been made public.
Syria asked Secretary-General Ban Ki-moon on March 21 to investigate an alleged chemical weapons attack by rebels two days earlier on Khan al-Assal village in northern Aleppo province. The rebels blamed regime forces for the attack.
The following day, Britain and France asked the U.N. chief to investigate allegations of chemical weapons use in two locations in Khan al-Assal and the village of Ataybah in the vicinity of Damascus, all on March 19, as well as in Homs on Dec. 23.
Syrian soldiers were reportedly killed and injured in the Khan al-Assal incident but the British and French believe this was the result of a misfired Syrian government shell, the diplomats and officials said.
After examining the letters from Syria and the Europeans, the secretary-general appointed a team of chemical weapons experts to investigate the allegations in Khan al-Assal and in Homs, where there was the most evidence. But the Syrian government has so far refused to allow the experts to go anywhere but Khan al-Assal.
The secretary-general said Wednesday that the team of experts, nonetheless, would proceed with "its fact-finding activities." He said additional information has been requested from the three governments.
U.N. diplomats say the chemical weapons experts are expected to visit camps for Syrian refugees, neighboring countries where Syrians have fled and possibly London and Paris to try to obtain information outside Syria. The diplomats spoke on condition of anonymity because details have not been announced.
British Foreign Secretary William Hague told Parliament last week that the government "is increasingly concerned that there is evidence of the use of chemical weapons in Syria."
"We welcome the UN secretary-general's announcement of an investigation into the allegations and again call on the Syrian regime to cooperate fully and allow the investigation unfettered access to all areas," he said. "They should take heed that the world is watching and those who order the use of chemical weapons or participate in their use must be held to account."
Director of National Intelligence James Clapper said Thursday the "increasingly beleaguered regime, having found that its escalation of violence through conventional means is not working, appears quite willing to use chemical weapons against its own people." But he would not yet confirm their use, in testimony before the Senate Armed Services Committee.
"We receive many claims of chemical warfare use in Syria each day, and we take them all seriously and we do all we can to investigate them," he told senators. He would not provide further details in the open hearing.
On Wednesday, Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, was asked by Sen. John McCain whether he is confident that U.S. forces could secure the chemical weapons caches within Syria.
"Not as I sit here today, simply because they've been moving it and the number of sites is quite numerous," Dempsey replied.
President Barack Obama has made clear that Assad would cross a red line if he were to use his suspected stockpile of chemical weapons — including nerve agents and mustard gas — against the Syrian people.
Syria is believed to have hundreds, if not thousands, of tons of chemical agents, said Leonard Spector, deputy director of the James Martin Center for Nonproliferation Studies in Monterey, California. This includes mustard gas, a blistering agent, and the more lethal nerve agents sarin and VX, he said.
The chemical agents are believed to be designed for use in artillery shells, aerial bombs and ballistic missiles, said Scott Stewart of the U.S. security think tank Stratfor.
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