* Inspectors won't deploy to Syria until access agreed
* Syria wants inspectors to focus only on Aleppo incident
* Britain, France want investigation of two other incidents
The United Nations and Syria have not yet agreed on how much access a team of chemical weapons inspectors will have to investigate allegations that such arms were used recently in the Syrian conflict, according to a letter to Syria's U.N. envoy.
The United Nations said last month it would investigate the Syrian government's allegations that rebels used chemical arms in an attack near the northern city of Aleppo.
Western countries also want a probe of two additional rebel claims about the use of such arms. The opposition says Syrian President Bashar al-Assad's government carried out all three alleged chemical attacks.
"There's no agreement on access yet," a U.N. Security Council diplomat told Reuters on condition of anonymity. "The inspectors won't be deploying until there's agreement on access and other modalities."
There has been an exchange of letters about access for the investigators between Syrian Ambassador Bashar Ja'afari and the head of the U.N. Office of Disarmament Affairs Angela Kane, according to a letter from Kane obtained by Reuters on Thursday.
That letter said Ja'afari wrote to Kane on Tuesday suggesting amendments to her proposed "legal and logistical parameters" for the investigation.
Ja'afari has said repeatedly that the inspectors need only limited access to the areas related to the Aleppo incident, in which the government and rebels accuse each other of firing a missile laden with chemicals that killed 26 people.
Diplomats said Assad's government has also suggested it wants a say in who will be on the inspection team.
Kane responded to Ja'afari by saying that it was "solely for the Secretary-General (Ban Ki-moon) to determine the composition of the investigation mission, which should have the necessary freedom of movement and access to conduct a thorough and objective investigation."
Kane made clear to Ja'afari that although the primary focus of the investigation would be the Aleppo incident, there were other alleged chemical weapons attacks to consider as well.
"We must remain mindful of the other allegations that chemical weapons were used elsewhere in the country," she wrote.
France and Britain wrote to Ban last month requesting that any investigation look into rebel allegations of an attack near Damascus, as well as one in Homs in late December. The rebels blame Syria's government for those incidents as well as the Aleppo attack.
The French and British request enraged Russian Ambassador Vitaly Churkin, who accused them of trying to "delay and possibly derail" the U.N. probe.
Russia has criticized Western and Arab calls for Assad to give up power and, together with China, has blocked three U.N. Security Council resolutions meant to pressure him to end the violence. Moscow has also differed with the West over which side was to blame for massacres and other atrocities in Syria.
Western diplomats and U.N. officials say Ban is determined to have all chemical weapons allegations investigated.
Ban has said he wants the inspection team, to be headed by Swedish scientist Ake Sellstrom, to deploy to Syria as soon as possible. There was no mention of a deployment date for the inspection team in Kane's letter.
"The United Nations is continuing to discuss the content of the exchange of letters with the government of Syria and is hopeful that a mutual understanding ... will be reached soon," said U.N. spokesman Eduardo del Buey.
Ja'afari did not respond immediately to a request for comment.
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 two-year-old Syrian conflict. The United Nations estimates the conflict has resulted in the loss of more than 70,000 lives.