Chinese President Xi Jinping has called for all sides to exercise restraint in a call with US President Donald Trump, as a nervous South Korea and Japan sought to join drills with a US aircraft carrier strike group headed for Korean waters.
Reclusive North Korea said at the weekend it was ready to sink the US aircraft carrier, the USS Carl Vinson, which Trump had ordered to waters off the Korean peninsula as a warning to the nuclear-armed North.
Japan said on Sunday it had sent two Japanese destroyers to join the carrier group for drills, and South Korea said it was also in talks about holding joint naval exercises.
China is increasingly worried the situation may spin out of control, leading to war and a chaotic collapse of its isolated and poverty-struck neighbour.
Xi told Trump that China resolutely opposes any actions that run counter to UN Security Council resolutions, a Chinese foreign ministry statement said.
China "hopes that all relevant sides exercise restraint, and avoid doing anything to worsen the tense situation on the peninsula", the statement paraphrased Xi as saying.
The nuclear issue can only be resolved quickly with all relevant countries pulling in the same direction, and China is willing to work with all parties, including the US, to ensure peace, Xi said.
Tensions have risen sharply in recent months, with Washington and its allies fearing Pyongyang could conduct another nuclear missile test or launch more ballistic missiles in defiance of United Nations sanctions.
North Korea celebrates the 85th anniversary of the foundation of its Korean People's Army on Tuesday, and has marked similar events in the past with nuclear tests or missile launches.
Earlier, Japanese Prime Minister Shinzo Abe described his conversation with Trump as a "thorough exchange of views".
"We agreed to strongly demand that North Korea, which is repeating its provocation, show restraint," Abe told reporters.
"We will maintain close contact with the United States, keep a high level of vigilance and respond firmly."
Abe also said he and Trump agreed that China, North Korea's sole major ally, should play a large role in dealing with Pyongyang.
The US government has not specified where the carrier strike group is, but US Vice President Mike Pence said on Saturday it would arrive "within days".
South Korean Defence Ministry spokesman Moon Sang-gyun gave no further details about the South's plans, other than saying Seoul was holding discussions with the US Navy.
"I can say the South Korean and US militaries are fully ready for North Korea's nuclear test," Moon said.