TOKYO, Jan 31 (Reuters) - NATO chief Jens Stoltenberg and Japanese premier Fumio Kishida pledged on Tuesday to strengthen ties, saying Russia's invasion of Ukraine and its growing military cooperation with China had created the most tense security environment since World War Two. The comments came in a statement issued during Stoltenberg's trip to Japan following a visit to South Korea on which he urged Seoul to increase military support to Ukraine and gave similar warnings about rising tension with China. See here for a complete list of exchanges and delays