U.S. officials said there was a higher-than-usual level of activity by Chinese bombers, signaling a possible heightened state of readiness by Beijing, reclusive North Korea's sole major ally, although the officials played down concern 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, an important anniversary that comes at the end of major winter military drills, South Korea's Unification Ministry spokesman Lee Duk-haeng said.
Top envoys from the United States, South Korea and Japan on North Korea are due to meet on Tuesday, South Korea's foreign ministry said, to "discuss plans to rein in North Korea's additional high-strength provocations, to maximize pressure on the North, and to ensure China's constructive role in resolving the North Korea nuclear issue".
South Korea and the United States have also been conducting annual joint military exercises, which the North routinely criticizes as a prelude to invasion.
"It is a situation where a lot of 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 on Thursday praised Chinese efforts to rein in "the menace of North Korea", 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 North Korea over its nuclear and missile programs.
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 signaled that they knew the precise reason for such activity.