US President Barack Obama landed in South Korea on Friday afternoon with security and the threat posed by a nuclear-armed North Korea at the top of his agenda - and just hours after a spokesman for the South Korean defense ministry warned that a new demonstration of the North's prowess in atomic weapons could come at any time.

"North Korea has completed all preparations to conduct a nuclear test suddenly," Kim Min-seok, the spokesman for Seoul's defense ministry, told reporters in a briefing.

"With all the preparations done, it is only a matter of political decision and strategic judgment for the North Korean leadership to conduct a nuclear test," he added.

Pyongyang first announced that it would carry out a "new form" of nuclear test in a statement issues in late March. Analysts suggested that it might be timed to coincide with the April 15 birthday of Kim Il Sung, the late founder of the nation and the grandfather of current ruler Kim Jong Un, or the April 27 anniversary of the end of the Korean War in 1953 - a date that the North still celebrates as "Victory Day."

Sign to Washington

It is now believed that a fourth nuclear test - after blasts in 2006, 2009 and 2013 - is more of a signal to the United States.

"The timing of a test - even if they go ahead with it in the next few weeks - is intentionally linked to Obama's visit to Seoul," Go Ito, a professor of international relations at Tokyo's Meiji University, told DW.

"This is what the North Korean regime does time after time, even if it seems nonsensical to outsiders, to deliberately provoke the US in this way," he added.

The expert believes that Kim Jong Un is fully aware that his military has no chance of victory in any conflict with the US and South Korea, but he is attempting to provoke Washington sufficiently for it to agree to hold talks.

Toshimitsu Shigemura, a professor at Tokyo's Waseda University and an expert on North Korean affairs, said that would be a victory for Pyongyang as it would bestow legitimacy on the regime and would be used in propaganda to insist that the US had bowed to Pyongyang's might.

Deepening isolation

That isolation has deepened in recent weeks with ties that appeared to be developing with South Korea once again stretched to breaking point, while even Pyongyang's traditional ally of China seems to be losing patience with the regime.

"Peace and stability are in the immediate interests of China," a spokesman for the foreign ministry in Beijing said on Thursday. "We will by no means allow war or chaos to occur on our doorstep."

Beijing is "encouraging all relevant parties to peacefully resolve issues through dialogue and consultation," Qin Gang said.

As an indication of China's displeasure with its belligerent neighbor, trade statistics show that no crude oil was exported to North Korea in the first three months of this year - an unprecedented move that is designed to show Pyongyang just how much it needs China's assistance.

Preparations continue

Analysts at the US-Korea Institute at the Johns Hopkins School of Advanced International Studies has released satellite images of North Korea's nuclear testing grounds, in the northeast of the country, that show an increase in preparatory work.

A tunnel for the nuclear device appears to have been excavated, although final preparations - including the installation of satellite dishes and communication facilities that were noted before the last test - have yet to be completed.

In a statement carried by the state-run Korean Central News Agency, the North's Committee for the Peaceful Reunification of Korea said Seoul "should not even dream that we will be coaxed into laying down our nuclear weapons."

Speaking in Tokyo shortly before he flew to South Korea on Friday, President Obama described North Korea as "dangerous" and promised to work with other nations in the region to apply more pressure on the regime.

"North Korea has engaged in provocative actions for the last several decades," the president said. "It's been an irresponsible actor on the international stage for the last several decades."

"So our message on North Korea has been consistent throughout," Obama added. "It is the most isolated country in the world. It is subject to more international sanctions and international condemnation than any country in the world.

"And what we have said is if you are, in fact, serious about North Korea being a normal nation, then you've got to start changing your behavior," he said. "And that starts with the basic principle of denuclearizing the Korean Peninsula."