(Adds North Korean media report) SEOUL, Jan 4 (Reuters) - North Korea said on Friday it would boost its war deterrent, a day after the United States said it was sending its nuclear envoy back to Asia to discuss an atomic disarmament deal on which Pyongyang has missed a deadline.North Korea failed to fully account for its nuclear weapons programme or answer U.S. suspicions that it has secretly tried to enrich uranium for weapons. It was supposed to have made the declaration by the end of 2007. "(We) will further strengthen our war deterrent capabilities in response to U.S....