In this pool photograph distributed by the Russian state agency Sputnik, Russia's President Vladimir Putin shakes hands with North Korea's leader Kim Jong Un during a meeting in Pyongyang on June 19

In this pool photograph distributed by the Russian state agency Sputnik, Russia's President Vladimir Putin shakes hands with North Korea's leader Kim Jong Un during a meeting in Pyongyang on June 19 / AFP via Getty Images

SEOUL, South Korea — Russia and North Korea signed a new treaty that provides mutual defense assistance, formalizing the two countries' heightened level of military and diplomatic cooperation.

President Vladimir Putin and North Korean leader Kim Jong Un signed the comprehensive strategic partnership treaty after their summit in Pyongyang on Wednesday.

In a press conference following the meeting, Putin said the agreement "includes the provision of mutual assistance in case of an aggression against one of the signatories."

North Korea previously had an alliance treaty with the Soviet Union that stipulated automatic involvement in case of an attack on either of the countries. After the fall of the Soviet Union, it was succeeded in 2000 by a lower-level treaty with Russia.

Putin called the new treaty, which would replace the previous ones, "a truly groundbreaking document that reflects the desire of the two countries not to rest on their laurels, but to bring our relations to a new qualitative level." He added it "sets large-scale tasks and benchmarks for deepening Russian-Korean relations in the long term."

Both Kim and Putin stressed that the new pact is "defensive" in nature.

Putin's visit to North Korea — the first in 24 years — comes as the two countries have grown increasingly close in the past few years since Russia invaded Ukraine. North Korea is suspected to have provided Russia with millions of artillery shells and other weapons for use in Ukraine, in exchange for food, energy and military technologies.

The full scope of the treaty and other agreements signed on Wednesday are not publicized. And some North Korea watchers question the sustainability of the current bilateral ties.

But the United States and its allies have raised concerns that a growing military partnership would embolden the two ostracized countries and destabilize the region and beyond.

In Washington on Tuesday, NATO Secretary General Jens Stoltenberg said after his meeting with Secretary of State Anthony Blinken that Russia's war is "propped up" by countries like North Korea and China. "If they succeed in Ukraine, it will make us more vulnerable and the world more dangerous," he said.

In a show of strong friendship, Kim personally greeted Putin on a red-carpeted tarmac at the Pyongyang International Airport well past 2 a.m. on Wednesday. Putin was originally scheduled to arrive on Tuesday for a two-day visit.

After hugs and a brief chat, the two rode in a Russian-made Aurus sedan together to the Kumsusan State Guesthouse along the capital's streets, which flew Russian national flags and welcoming banners.

North Korea put on an extravagant welcoming ceremony later on Wednesday at the Kim Il Sung Square, which was filled with military bands, honor guards and North Korean citizens waving flowers and flags under sweltering heat.

In this pool photograph distributed by the Russian state agency Sputnik, people release balloons in the air as Vladimir Putin and Kim Jong Un attend a welcoming ceremony at Kim Il Sung Square in Pyongyang on June 19. Putin enjoyed a red carpet welcome, a military ceremony and an embrace from North Korea's Kim Jong Un during the state visit.

In this pool photograph distributed by the Russian state agency Sputnik, people release balloons in the air as Vladimir Putin and Kim Jong Un attend a welcoming ceremony at Kim Il Sung Square in Pyongyang on June 19. Putin enjoyed a red carpet welcome, a military ceremony and an embrace from North Korea's Kim Jong Un during the state visit. / AFP via Getty Images

At the expanded meeting that followed, Kim Jong Un said, "The situation globally is now changing fast as it becomes more challenging. Against this background, we are set to strengthen strategic interaction with Russia, with the Russian leadership as we go forward," according to Russia's Tass news agency.

Kim was also quoted as saying that his country "appreciates the important mission and role of strong Russia in maintaining strategic stability and balance in the world."

He further promised "full support and solidarity with the Russian government, army and people in carrying out the special military operation in Ukraine to protect the sovereignty, security interests and territorial integrity," according to Sputnik.

Putin thanked Kim for North Korea's support for the war and stressed the two countries' historical comradeship spanning from the early 20th century to their current opposition to "the hegemony policy, the imperialist politics of the United States and its satellites," Tass reported.

In the press conference later, Putin criticized the U.S.' "confrontational policy" in the region and defended North Korea against "all the attempts to place the blame for destabilization of the international situation" on his host. He called on the United Nations Security Council to review the sanctions it places on North Korea.

He also suggested that the two countries can engage in advanced military-technical cooperation, in addition to increasing exchanges in trade, culture, tourism, education, and agriculture.