With ballistic missiles regularly flying nearby, Japan and South Korea need little reminder of the threat that North Korea and its nuclear arsenal poses to its neighbors. But the stunning revival of a Cold War-era mutual defense agreement during a visit this week by President Vladimir V. Putin of Russia to Pyongyang, the capital, amped up the pressure on some of the hermit kingdom’s closest neighbors.

Mr. Putin and North Korea’s leader, Kim Jong-un, agreed that if one country found itself in a state of war, then the other would provide “military and other assistance with all means in its possession without delay,” according to the text of the agreement released Thursday by the North’s official Korean Central News Agency.

Analysts were still sorting through the text of the agreement to understand how far it would extend, either in terms of Mr. Putin’s war in Ukraine or any future conflict on the Korean Peninsula. But the pledge, along with indications that Russia could help bolster North Korea’s continuing quest to build its nuclear capabilities, rattled officials in Tokyo and Seoul.

Mr. Kim has grown increasingly hostile toward South Korea and this year abandoned a longtime goal of reunifying with the South, however unlikely that might have been. Now he describes the South solely as an enemy that must be subjugated, if necessary, through a nuclear war. And he has often tested his ballistic missiles by flying them toward Japan, demonstrating North Korea’s provocative stance toward its former colonizer.

Mr. Kim’s alliance with Mr. Putin, analysts said, would escalate tensions in northeast Asia by sharpening a divide between the democratic partnership among the United States, South Korea and Japan on the one side, and the autocratic camp of Russia, North Korea and China on the other.

“It is bad news for international efforts to prevent North Korea from advancing its nuclear and missile technologies,” said Koh Yu-hwan, former head of the Seoul-based Korea Institute for Unification Studies.

Mr. Putin’s protracted war in Ukraine has led him to deepen relations with Mr. Kim. U.S. and South Korean officials say he has sought and received Soviet-grade ammunitions from Pyongyang — accusations that both Moscow and Pyongyang deny.

The war in Ukraine has loomed large in the region. “The Ukraine of today may be the East Asia of tomorrow,” Prime Minister Fumio Kishida of Japan has often said.

Officials in Seoul and Tokyo were quick to point out that if Russia sends any military technology to North Korea, it would violate United Nations Security Council resolutions that it has previously signed.

“We are seriously concerned about the fact that President Putin did not rule out military-technical cooperation with North Korea,” Yoshimasa Hayashi, Mr. Kishida’s chief cabinet secretary, said at a press briefing in Tokyo.

In some respects, the meeting between the two authoritarian leaders, both desperate for outside support, provided a bit of an I-told-you-so moment for the United States and its Asian allies, who have been preparing in recent years for growing security challenges from North Korea as well as China, and sometimes have faced domestic political headwinds for doing so.

Japan has vowed to increase its defense budget and pushed limits on what it could do under its pacifist Constitution, including purchasing more fighter jets and tomahawk missiles. After years of frosty relations, Mr. Kishida and President Yoon Suk Yeol of South Korea agreed to strengthen bilateral ties between their two countries and have drawn closer in a three-way partnership with the U.S. to forge mutual security arrangements. Over the last year, the three countries have participated in more than 60 trilateral diplomatic meetings, military exercises and intelligence sharing, according to the U.S. Embassy in Tokyo.

“I think it shows how prescient President Biden, President Kishida and President Yoon were to spend political capital,” Rahm Emanuel, the U.S. ambassador to Japan, said in an interview. “It was prescient not just from a political standpoint, but from a strategic standpoint because now Russia and North Korea” may be developing weapons together.

The revival of a Cold War-era mutual defense pledge between North Korea and Russia in this fraught global moment spooked other countries in the region.

“What I think is more dangerous is that it shows that the relationship will be more long term than perhaps we initially thought and that it may be more strategic than transactional,” said Bruce Klingner, a senior research fellow in Asian studies at the Heritage Foundation in Washington. “We don’t know the parameters of how far each country will go in support of each other.”

At the very least, it shows that Russia is willing to flagrantly dismiss U.N. sanctions.

“It was not that long ago that Russia was backing U.N. sanctions on North Korea,” said James D.J. Brown, a professor of political science at the Tokyo campus of Temple University who specializes in relations between Russia and East Asia. “So it confirms that Russia is not only not implementing sanctions themselves but actively undermining them and will help North Korea to evade sanctions.”

In Seoul, the meeting between Mr. Putin and Mr. Kim was likely to revive discussion of whether South Korea should consider arming itself with nuclear weapons as well as start anticipating what might happen if Donald Trump is re-elected president of the United States.

“It is time for South Korea to have a fundamental review of its current security policy, which depends almost totally on the U.S. nuclear umbrella to counter the North Korean nuclear threat,” said Cheong Seong-chang, the director of the Center for Korean Peninsula Strategy at the Sejong Institute.

In one respect, the growing bond between Russia and North Korea could help cement the recently revived ties between Tokyo and Seoul as well as their three-way cooperation with the United States. Many analysts have worried that a change of administration in either the United States or South Korea could endanger these relationships. (Japan is considered relatively stable.)

“In some ways it sets up the justification to continue trilateralism after potentially a Trump administration comes in or if progressives come in Korea,” said Jeffrey Hornung, a senior political analyst who specializes in Japan at the RAND Corporation in Washington. “Even though it doesn’t change what Seoul or Tokyo should be doing, it definitely adds a new factor of what they have to consider.”

But an editorial in Hankyoreh, a left-leaning daily newspaper in Seoul, questioned the wisdom of close cooperation among the United States, Japan and South Korea, saying it had put South Korea “consistently in conflict with China and Russia, two countries with a huge influence on the Korean Peninsula’s political situation. It’s time to reflect on whether this skewed approach to diplomacy hasn’t had the effect of contributing to the development of relations between North Korea and Russia.”

Despite the drama in Pyongyang this week, some analysts said that the biggest worry for the region remains the rising military ambitions of China.

“The maritime buildup in the East China Sea or South China Sea or in space and cyber and a multi-domain war capability — they all justify our new policy,” said Kunihiko Miyake, a former Japanese diplomat and a special adviser at the Canon Institute for Global Studies in Tokyo. Mr. Putin’s visit to North Korea, he said , “is just another example, and not the biggest example” of threats in Asia.