U.S. National Security Adviser John Bolton said on Thursday he planned to use a trip to Belarus to warn its leader, President Alexander Lukashenko, of the security threat posed by Russia. Bolton will be the most senior U.S. official in years to visit Belarus, a move that comes as Moscow and Minsk are moving to closer integrate their countries as part of a union state project that has fuelled fears of a quiet annexation by Moscow. Russia views Belarus as a buffer between its western border and Europe as ties with the West have sunk to post-Cold War lows, but it denies there is anything untoward going on with its union state project and says Belarus is a close and valued ally.