U.N. talks with Syria on chemical arms probe at impasse UNITED NATIONS (Reuters) - Discussions between the United Nations and Syrian President Bashar al-Assad's government on a possible investigation into the alleged use of chemical weapons in Syria have reached an impasse, U.N. diplomats said on Wednesday. Syria and the United Nations have been exchanging letters for weeks but the two sides are far from agreement on how the investigation should be run, diplomats said on condition of anonymity. Syria has asked the United Nations only to investigate what it says was a rebel chemical attack near Aleppo last month. The opposition has blamed President Bashar al-Assad's forces for that strike and also wants the U.N. team to look into other alleged chemical attacks by the government. There have been three alleged chemical weapons attacks - the one near Aleppo and another near Damascus, both in March, and one in Homs in December. The rebels and Assad's government blame each other for all of them. So far, the Syrians are refusing to let inspectors go anywhere but Aleppo, while the United Nations is insisting that the team goes to both Aleppo and Homs. France and Britain wrote to U.N. Secretary-General Ban Ki-moon last month saying the mission should look into all three cases. The Syrian Foreign Ministry restated that position in a news release on Monday, saying the U.N. request to go anywhere in Syria where chemical weapons may have been used was not in keeping with the Syrian government's original request. In an April 6 letter from Syrian Foreign Minister Walid al-Moualem to Ban, obtained by Reuters, Assad's government said the inspectors should go first to Aleppo and if they are seen to be impartial, the possibility of visiting Homs could be discussed. "After the mission completes its work, and ascertaining its honesty and neutrality and the credibility of its work away from politicization, it may be possible to look into the Homs claims," the letter said. Moualem also complained about the leak of previous letters exchanged between Syria and the United Nations to Reuters, saying it "left the impression of a lack of seriousness on the part of the (U.N.) secretariat on cooperation in good faith." The United Nations said it was studying a recent Syrian letter, although it was not immediately clear if that letter was Moualem's or a more recent one. Moualem offered Syrian planes that would have the U.N. logo painted on them "to ensure the safety of the (inspection) team members in view of the prevailing security situation." Western delegations said the Syrian response of April 6 was unacceptable and that the chemical weapons team must have assurances now that it can visit both Aleppo and Homs. After meeting in The Hague with the head of the Organization for the Prohibition of Chemical Weapons, or OPCW, which is providing scientists and equipment for the inspection team, Ban said an advance team was in Cyprus, ready to go to Syria within 24 hours. Britain, France and the Americans have given Ban information about the possible use of chemical weapons in Aleppo and Homs, U.N. diplomats said. "He (Ban Ki-moon) recognized that there is sufficient evidence to investigate both in Homs and in Aleppo," the senior diplomat said. "They should not go in to investigate the one incident if they are told by the Syrians that they can't investigate the second incident," the diplomat said. So we would hope that the U.N. would not do that. The United Nations has two options, diplomats said, if Syria refuses to promise the mission can visit Homs, starting with Ban reporting back to U.N. member states that the Syrians are not cooperating. "Or you can continue the investigation but outside Syria in terms of investigating witnesses in the camps," the senior diplomat said. There may be some physical evidence of people who have been poisoned (who are now) outside Syria. An earlier exchange of letters between Syria's U.N. ambassador, Bashar Ja'afari, and U.N. disarmament chief Angela Kane highlighted other conditions Assad's government wants on the inspections, U.N. diplomats said on condition of anonymity. Ja'afari insisted on appointing an observer to accompany the inspection team and wanted duplicates of any samples taken to test for chemical weapons traces, the diplomats told Reuters. There will be at least 15 members of the inspection team, mainly from Nordic countries, Latin America or Asia. None of them is from a permanent member of the U.N. Security Council. According to Western intelligence agencies, Syria is believed to have one of the largest remaining stockpiles of undeclared chemical weapons in the world. If it goes ahead, the investigation will try to determine only if chemical weapons were used, not who used them. If it is confirmed that the weapons were used, it would be the first time in the 2-year-old Syrian conflict. The United Nations estimates the Syrian conflict has resulted in more than 70,000 deaths. Additional reporting by Sami Aboudi in Dubai; Editing by Bill Trott and Peter Cooney