The warnings came Sunday after U.S. Secretary of State John Kerry spoke on the telephone with Foreign Ministers Fumio Kishida of Japan and Kim Sung-hwan of South Korea, CNN reported, quoting a summary of the calls provided by the U.S. State Department.
The three agreed the North must understand "it will face significant consequences from the international community if it continues its provocative behavior," the State Department said.
North Korea has been signaling its intentions to conduct its third nuclear test after the U.N. Security Council approved a resolution last month to tighten existing sanctions against the Communist country for its December long-range rocket firing to place a satellite in space in violation of the sanctions. North Korea conducted its previous two nuclear tests in 2006 and 2009.
The United States and South Korea are conducting joint naval exercises off the Korean Peninsula this week.