(Adds al-Nusra quote, detail) BEIRUT, Feb 22 (Reuters) - A Syrian truce being coordinated by the United States and Russia would be for an initial two weeks and could be extended, a Syrian opposition official said on Monday. "The length of the proposed truce is two weeks, but it could be extended indefinitely if the parties commit to it," said Khoja, who is president of the Syrian National Coalition and a member of the opposition High Negotiations Committee, which is backed by Saudi Arabia.