MOSCOW/ASTANA (Reuters) - A Moscow-backed congress of all Syria s ethnic groups may take place in Russia and begin working on a new constitution as early as next month, the news agency RIA reported on Monday, citing a source familiar with the situation. The congress, which President Vladimir Putin first mentioned earlier this month, may take place in mid-November at Russia s  Black Sea resort of Sochi, RIA said. The idea of a congress had United Nations backing, a senior Russian negotiator on Syria said. Russia s Hmeymim air base in Syria also might be used, he added.  This matter is still being discussed,  Alexander Lavrentyev, the head of the Russian delegation at Syria talks in Kazakhstan, told reporters between meetings with diplomats from Turkey and Iran.  As you know (U.N. Special Representative on Syria Staffan) de Mistura has in principle supported the idea of holding the congress,  Lavrentyev said.  Although he had some reservations, he supported this initiative of Russia.  Lavrentyev said the congress would focus on seeking  compromise solutions towards the political settlement  of the Syrian conflict. Russia, Turkey and Iran are holding the seventh round of talks on Syria - which are separate from the U.N.-sponsored Geneva process - in Astana, the capital of Kazakhstan, this week. 