WASHINGTON (Reuters) - U.S. national security adviser Robert O’Brien will represent the United States at back-to-back virtual summits with Asian countries this weekend, the White House said on Friday.O’Brien will lead the U.S. delegation in talks with the 10 members of the Association of Southeast Asian Nations (ASEAN) beginning on Friday evening U.S. time, or Saturday morning in Asia. He will participate in a broader East Asia Summit on Sunday, when 15 Asia-Pacific economies - excluding the United States - are expected to sign a China-backed Regional Comprehensive Economic Partnership, which could become the world’s largest free trade agreement. The White House has not yet said who will represent the United States at a leaders’ meeting of a larger grouping of Pacific Rim nations, the Asia-Pacific Economic Cooperation (APEC) forum, next Friday.