(Adds details, context)
MOSCOW, March 28 (Reuters) - Russia said on Thursday that the Arab League's decision to give a seat to the Syrian opposition at its recent summit cast doubt on the mandate of U.N.-Arab League peace mediator Lakhdar Brahimi.
Opposition leader Moaz Alkhatib took Syria's vacant seat on Tuesday at the Arab summit, which also has lent its support to giving military aid to rebels fighting President Bashar al-Assad. Russia, a long-time arms supplier to Damascus, has vehemently opposed arming the rebels.
"A huge question emerges as regards the mandate of Lakhdar Brahimi, who until the summit was the U.N. and Arab League representative for promoting contacts and talks between the government and opposition," Foreign Minister Sergei Lavrov said.
"I just don't see how Mr. Brahimi can continue to be considered the representative not just of the United Nations but of the Arab League," Lavrov told a joint briefing after talks with Thai Foreign Minister Surapong Tovichakchaikul.
Russia has supported Brahimi, who has met in recent months with Russian and U.S. officials as part of an effort to end the violence that killed more than 70,000 people in two years.
Brahimi and U.N. Secretary General Ban Ki-moon voiced regret over the lack of progress earlier this month. Russia and China have blocked three resolutions in the U.N. Security Council, and Moscow says Assad's exit from power must not be a precondition for peace talks. (Writing by Steve Gutterman; Editing by Rosalind Russell)