* Russia calls for investigation into alleged chemical attack
* Says rocket was released from area controlled by opposition
* Alleged attack could be "provocation" (Adds details, quotes)
MOSCOW, Aug 21 (Reuters) - Russia's Foreign Ministry called for a thorough investigation on Wednesday into reports that Syrian government forces had launched a chemical attack, suggesting that rebels could have staged the assault to provoke international action.
Syria's opposition accused President Bashar al-Assad's forces of gassing many hundreds of people - by one report as many as 1,300 - on Wednesday in what would, if confirmed, be the world's worst chemical weapons attack in decades.
Foreign Ministry Spokesman Alexander Lukashevich said that its sources in Syria said that a homemade rocket carrying unidentified chemical substances had been launched from an area controlled by the opposition.
"All this cannot but suggest that once again we are dealing with a pre-planned provocation," Lukashevich said in a statement.
"This is supported by the fact that the criminal act was committed near Damascus at the very moment when a mission of U.N. experts had successfully started their work of investigating allegations of the possible use of chemical weapons there," he said.
Russia has protected Assad during the more than two-year conflict in Syria, vetoing U.N. sanctions aimed at pressuring him to end violence.
Moscow has also demanded that a U.N. investigation into the alleged use of chemical weapons also look into possible use by rebels.
"Moscow considers important an objective and professional investigation into what happened. And we call on all those who have the possibility to influence armed extremists make every effort to end provocations with chemical agents," Lukashevich said. (Reporting By Thomas Grove; Editing by Raissa Kasolowsky)