South Korea said on Friday it was on heightened alert in advance 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.

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 military drills, South Korea's Unification Ministry spokesman Lee Duk-haeng said.

The South 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 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-US military drills," Lee said.

"We are closely watching the situation and will not be letting our guards down."

READ MORE: North Korea on 'maximum alert' after US threats

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. China's foreign ministry, however, denied reports that bomber aircraft were on a heightened state of alert.

In Russia, meanwhile, a Kremlin spokesman declined to comment on media reports that Russia was moving military hardware and troops towards the border with North Korea.

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 US and China have warned against.

Top envoys from the United States, South Korea, and Japan on North Korea are 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".

US President Donald Trump386 said if China wants to solve "the North Korean problem, they will". Trump has been pressing Beijing to ramp up pressure on Pyongyang over its nuclear ambitions.

In a tweet on Friday morning, Trump said China is the "economic lifeline" to North Korea. He conceded that "nothing is easy" regarding the tense situation.

US defence chief Jim Mattis said on Friday that North Korea's rhetoric was provocative, but he had learned not to trust it.

'Unusual moves'

An official Chinese newspaper said there was optimism about persuading the North to end its pursuit of its nuclear programme without the use of force, "now that even the once tough-talking Donald Trump is on board 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.

In Russia, some media have quoted residents as saying they have seen military hardware being moved towards North Korea, but Kremlin spokesman Dmitry Peskov said deployment of Russian troops inside Russia was not a public matter.

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 US with a nuclear missile.

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North Korea has said it would test missiles when it sees fit and a South Korean analyst said he believed they would do so.

"Without crossing the red line such as a nuclear test or a test launch of an intercontinental ballistic missile, until the April 25 anniversary of the Korean People's Army, North Korea is expected to continue to launch mid-range missiles," said Cheong Seong-chang, a senior research fellow at Sejong Institute outside Seoul.

The joint US-South Korea military exercises are due to finish at the end of April.

A US aircraft carrier strike group, led by the USS Carl Vinson, is heading towards the Korean peninsula, Trump's administration has said.