South Korea is 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.
US 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 concern and left open a range of possible reasons.
In Russia, a Kremlin spokesman declined to comment on media reports that Russia was moving military hardware and troops towards the border with North Korea, the RIA news agency quoted him as saying.
US 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 maximise pressure on the North, and to ensure China's constructive role in resolving the North Korea nuclear issue".
South Korea and the US have also been conducting annual joint military exercises, which the North routinely criticises as a prelude to invasion.
US 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".
US Defense Secretary Jim Mattis said on Friday North Korea's rhetoric was provocative but he had learned not to trust it.