By Stephen Adler and Timothy Heritage MOSCOW (Reuters) - Russia hopes an international peace conference on Syria will be held before the end of this year, Prime Minister Dmitry Medvedev said, despite reported differences with the United States over opposition representation. He appealed to both sides in Syria's civil war to compromise and criticized the opposition for demanding assurances of President Bashar al-Assad's departure as a condition for the talks. We're pushing them towards this, and I hope everyone who talks to different circles in Syria will do the same," he said. "It's a difficult process and everyone must compromise, including opposition leaders and the Syrian government, of course." Russia has been Assad's most powerful backer during the two-and-a-half-year-old conflict, delivering weapons, blocking three U.N. Security Council resolutions meant to pressure him and saying his exit cannot be a precondition for peace talks.