TOKYO (Reuters) - U.S. Secretary of State Rex Tillerson stressed on Thursday the importance of the United States working with allies Japan and South Korea to counter perceived threats from North Korea. “Trilateral cooperation is critical as we address the issue of North Korea,” Tillerson told Japanese Foreign Minister Fumio Kishida in Tokyo at the start of his first trip to Asia as President Donald Trump’s top diplomat. He visits South Korea and China later in the week. In addition to reassurances on dealing with Pyongyang’s repeated missile and nuclear tests, the former oil executive is also expected to press China to help restrain North Korea, while America’s allies will be looking to see how the new U.S. administration deals with Beijing’s growing economic and military clout.  