* U.N. first to probe alleged Aleppo chemical attack
* Western countries raise two other incidents
* Rebels blame all three on government (Adds U.N. letter, Russian envoy, diplomats)
By Steve Gutterman and Louis Charbonneau
MOSCOW/UNITED NATIONS, March 25 (Reuters) - Russia said on Monday that Western nations had muscled U.N. chief Ban Ki-moon into broadening an investigation into the alleged use of chemical arms in Syria and accused them of trying to use the probe to push President Bashar al-Assad from power.
The United Nations said last week it would investigate Syrian allegations that rebels used chemical arms in an attack near the northern city of Aleppo, but Western countries sought a probe of all claims about the use of such banned arms.
The Syrian request for an investigation relates to an alleged chemical attack near Aleppo. France and Britain wrote to Ban on Thursday to draw his attention to a second alleged attack near Damascus, as well as one in Homs in late December. The rebels blame Syria's government for all three incidents.
If an investigation adds credibility to the rebels' claims that the government has used chemical weapons, it would represent another blow to Assad's efforts to retain power.
Ban, the U.N. secretary-general, made clear on Thursday that the investigation would initially focus on the Aleppo incident, in which the government and rebels accuse each other of firing a missile laden with chemicals, killing 26 people. But he has made clear that he intends to broaden the investigation's mandate.
The Russian Foreign Ministry, however, said in a statement that Ban had succumbed to Western pressure to widen the investigation.
It said that in a letter to the U.N. Security Council, "Ban Ki-moon, under pressure from Western members of the council, took the unjustified step of broadening the mandate of the mission, trying to set as its task the investigation of all other supposed cases of chemical weapons use in Syria."
"We consider this approach counterproductive," the statement said, adding that there was no information pointing to any other incidents of the use of chemical weapons in Syria.
Ban's letter to the council on Friday, which was seen by Reuters, did not call for immediately broadening the investigation. But it did say that Ban had asked Britain, France and Syria for further information on the other alleged chemical attacks "with a view to verifying any alleged use of chemical weapons in Syria."
Several diplomats told Reuters on condition of anonymity that Russia was so annoyed with Ban's suggestion that he intended to broaden the scope of the investigation that the Russian mission asked Ban to withdraw his letter.
If Ban expanded the mandate to include other alleged chemical weapon attacks, several council diplomats and U.N. officials said he would need the consent of Assad's government if such an investigation were to take place.
"Realistically speaking, Assad would have to agree to this and it's hard to see him doing that," a diplomat said.
A spokesman for the Russian mission did not have an immediate comment.
DELAY AND DERAIL?
Russia also said the selection of an investigation team must be "apolitical and unprejudiced" and said Moscow believes Russian and Chinese experts must be included.
In separate remarks in Twitter, Deputy Foreign Minister Gennady Gatilov said the team "must without fail include representatives of the five permanent U.N. Security Council members, including Russian and Chinese chemical specialists."
Ban's Thursday announcement about an investigation followed a dispute between Russia and Western council members over the scope of the investigation. The five permanent council members are Russia, China, the United States, France and Britain.
After France and Britain wrote to Ban to draw his attention to a second alleged attack near Damascus and one in Homs in December, both of which rebels blame on the government, Russia accused them of trying to delay the inquiry.
Russian U.N. Ambassador Vitaly Churkin repeated his criticism of London and Paris on Monday, saying their call for a broader investigation was an attempt to "delay and possibly derail" the U.N. probe.
U.S. and European officials say there is no evidence of a chemical weapons attack. If one is confirmed, it would be the first use of such weapons in the two-year-old Syrian conflict, which the United Nations says has cost 70,000 lives.
Moscow initially accused rebels of using chemical weapons in the Aleppo incident, echoing the Syrian government version, but Gatilov later said it was unclear exactly what happened.
Russia has criticized Western and Arab calls for Assad to leave power and, together with China, has blocked three U.N. Security Council resolutions meant to pressure him to end violence. It has also differed with the West over which side was to blame for alleged massacres and other atrocities in Syria.
Damascus has not confirmed that it has chemical weapons, but says if it had them it would not use them on its own people.
Israeli Major General Yair Golan, who commands forces along the Syrian and Lebanese fronts, has said Syria's chemical arsenal is still under the control of the Damascus government.
"(Syria's) chemical weapons today are 100 percent under control. We can say this is good news for now, if you can call it good news," the head of Israel's northern command told the Israel Hayom newspaper in an interview published on Monday.
Israel, which occupies Syria's Golan Heights, has been on alert for spillover from the conflict next door. It said its forces had fired into Syria on Sunday, destroying a machinegun position that had shot at Israeli soldiers. (Reporting by Steve Gutterman in Moscow and Ari Rabinovitch in Jerusalem; Editing by Alistair Lyon, Stephen Powell and Cynthia Osterman)