Top diplomats from South Korea, the United States and Japan have vowed to take a stronger and more coordinated stance against Pyongyang.During a press briefing at the annual ASEAN Regional Forum in Myanmar on Sunday, South Korea's foreign minister Yun Byung-se characterized the situation on the Korean peninsula as "unstable" because of the growing threats from Pyongyangand called on the three countries to come up with a joint strategy to counter the threats.North Korea has been ratcheting up tensions in recent weeks by firing off short-range missiles, despite fresh condemnation from the United Nations Security Council.It even threatened to go ahead with a fourth nuclear test earlier this week.U.S. Secretary of State John Kerry also echoed the call for cooperation during a one-on-one meeting with Yun prior to the trilateral talks, where he agreed to continue to cooperate in making practical advances toward the denuclearization of North Korea, according to an official at Seoul's Foreign Ministry.Japanese Foreign Minister Fumio Kishida said, given the changing security environment, it's crucial that the three countries ramp up their cooperation.Kishida also ensured transparency with regard to Japan's recent negotiations with North Korea on Pyongyang's abductions of Japanese citizensShin Se-min, Arirang News.