(Adds undetermined activity at nuclear test site)
* South Korea says North can conduct provocation "any time"
* Trump speaks of "unusual moves"
By Ju-min Park and Ben Blanchard
SEOUL/BEIJING, April 21 (Reuters) - South Korea was on heightened alert on Friday 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.
North Korea will mark the 85th anniversary of the foundation of its Korean People's Army on Tuesday, as it concludes winter military drills at the same time as U.S. and South Korean exercises running until the end of April, South Korea's Unification Ministry spokesman Lee Duk-haeng said.
"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," he said, adding that South Korea would not let its guard down.
North Korea said on Friday the state of affairs on the Korean peninsula was "extremely perilous" because of "madcap American nuclear war manoeuvres aimed at trampling on our sovereignty and right to survival."
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. U.S. President Donald Trump has vowed to prevent North Korea from being able to hit the United States with a nuclear missile.
South Korean acting President Hwang Kyo-ahn told top officials on Thursday that given the April 25 anniversary, there were concerns that North Korea "can make another provocation again at any time."
New satellite imagery analysed by 38 North, a Washington-based North Korea monitoring project, found some activity under way at North Korea's Punggye-ri nuclear test site, but said it was unclear whether the site was in a "tactical pause" ahead of another test or was carrying out normal operations.
"Regardless, satellite imagery continues to indicate that the Punggye-ri nuclear test site appears able to conduct a sixth nuclear test at any time once the order is received from Pyongyang," 38 North said in an analysis on Friday.
U.S. officials said there was a higher-than-usual level of activity by Chinese bombers, signalling a possible heightened state of readiness by reclusive North Korea's sole major ally, although the officials played down concern and left open a range of possible reasons. Beijing denied its aircraft were on an increased level of alert.
The Russian military on Friday denied media reports that it was building up its forces near the border with North Korea, the Interfax news agency reported, citing an army spokesman. Moscow said military movements seen by residents there were part of pre-planned exercises.
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 the United States, China and others have warned against.
On Thursday, Trump 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."
North Korea's Foreign Ministry said in a statement that its military was ready to respond to American aggression.
"Now that we possess mighty nuclear power to protect ourselves from U.S. nuclear threat, we will respond without the slightest hesitation to full-out war with full-out war and to nuclear war with our style of nuclear strike, and we will emerge victor in the final battle with the United States."
Trump told a news conference on Thursday, "Some very unusual moves have been made over the last two or three hours. He said he was confident Chinese President Xi Jinping would "try very hard" to pressure 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.
In a tweet on Friday, Trump said: "China is very much the economic lifeline to North Korea so, while nothing is easy, if they want to solve the North Korean problem, they will."
Top U.S., South Korean and Japanese envoys are due to meet on Tuesday, South Korea's Foreign Ministry said, to "to maximize pressure on the North, and to ensure China's constructive role."
China's Defence Ministry said its forces on the border with North Korea were maintaining a state of normal combat preparedness and training.
Asked about Trump's comments, Chinese Foreign Ministry spokesman Lu Kang said Xi and Trump had had a full discussion about North Korea when they met this month.
"I can only say that via deep communications between China and the U.S. at various levels including at the highest levels, the U.S. now has an even fuller and more correct understanding of China's policy and position and has a more rounded understanding of China's efforts," Lu said. "We feel very gratified about this."
An official Chinese newspaper said there was optimism about persuading North Korea 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.
North Korea has said it will conduct nuclear and missile tests when it sees fit.
North Korea test-fired what the United States believed was a mid-range missile on Sunday. It blew up almost immediately. (Additional reporting by Polina Devitt in Moscow, Idrees Ali in Tel Aviv and David Brunnstrom in Washington; Writing by Jack Kim; Editing by Alistair Bell and Leslie Adler)