South Korea said on Friday it was on heightened alert ahead of another important anniversary in North Korea, with a large concentration of military hardware amassed on both sides of the border amid concerns about a new nuclear test by Pyongyang.
U.S. officials said there was a higher-than-usual level of activity by Chinese bombers, signalling a possible heightened state of readiness by Beijing, reclusive North Korea's sole major ally, although the officials played down concerns and left open a range of possible reasons.
U.S. and South Korean officials have been saying for weeks that the North could soon stage another nuclear test in violation of United Nations sanctions, something both the United States and China have warned against.
North Korea marks the 85th anniversary of the foundation of its Korean People's Army on Tuesday, a key anniversary that comes at the end of major winter military drills, South Korea's Unification Ministry spokesman Lee Duk-haeng said.
South Korea and the United States have also been conducting annual joint military exercises, which the North routinely criticises as a prelude to invasion.
"It is a situation where a lot of (military) exercise equipment is amassed in North Korea and also a lot of strategic assets are situated on the Korean peninsula because of the South Korea-U.S. military drills," Lee told a briefing.
"We are closely watching the situation and will not be letting our guards down," Lee said.
U.S. President Donald Trump praised Chinese efforts to rein in "the menace of North Korea" on Thursday, after North Korean state media warned the United States of a "super-mighty preemptive strike".
Trump told a news conference "some very unusual moves have been made over the last two or three hours", and that he was confident Chinese President Xi Jinping would "try very hard" to pressure Beijing's neighbour North Korea over its nuclear and missile programmes.
Trump gave no indication of what the moves might be. None of the U.S. officials who told Reuters about the heightened level of activity by Chinese bombers suggested alarm or signalled that they knew the precise reason for such activity.
An official Chinese newspaper said on Friday there was optimism about persuading the North to end its pursuit of a nuclear programme without the use of force, "now that even the once tough-talking Donald Trump is onboard for a peaceful solution".
"Beijing has demonstrated due enthusiasm for Washington's newfound interest in a diplomatic solution and willingness to work more closely with it," the state-run China Daily said in an editorial.
Tensions have risen sharply in recent months after North Korea conducted two nuclear weapons tests last year and carried out a steady stream of ballistic missile tests. Trump has vowed to prevent North Korea from being able to hit the United States with a nuclear missile.
The isolated country's military will mark the 85th anniversary of the founding of the Korean People's Army on Tuesday amid speculation Pyongyang will use the occasion to test-launch a ballistic missile or conduct a nuclear test.
The joint U.S.-South Korea military drills are due to finish at the end of April, and a U.S. aircraft carrier strike group, led by the USS Carl Vinson, is also heading towards the Korean Peninsula, Trump's administration has said.