North Korean leader Kim Jong-un was on his way back to Pyongyang on Sunday, wrapping up a six-day trip to the Russian Far East that focused on military cooperation with Moscow amid blistering criticism from the international community for defying sanctions.

As his meeting with Russian President Vladimir Putin stoked fears across the West and its allies over a potential arms deal, South Korean President Yoon Suk Yeol said the military cooperation between Pyongyang and Moscow is "illegal and unjust" and infringes UN Security Council resolutions and many other international sanctions.

Yoon also said in a written interview with AP published hours before Kim's departure from Russia, that any nuclear attack by North Korea will bring about the demise of the regime.

"Going forward, (South Korea)-US extended deterrence will develop into a joint system in which both countries discuss, decide and act together," he said. "We will also enhance the ability to deter and respond to any nuclear or missile threat from North Korea."

Kim's armored private train departed from the Artyom railway station in the far eastern Primorsky region after he was greeted by Russian officials at a farewell ceremony, according to videos distributed by Russian state media outlets.

Kim's stay in Russia for five nights and six days since arriving at the border station of Khasan on Sept. 12 marks his longest foreign trip. In his seven foreign visits since 2018, his stays have typically been as short -- such as one night and two days, occasionally extending to four nights and five days in the host country.

Kim received five explosive drones, a reconnaissance drone, and a bulletproof vest as gifts from Oleg Kozhemyako, the governor of Russia's Far Eastern Primorsky region.

TASS said the "leader of the DPRK received five kamikaze drones and a 'Geran-25' reconnaissance drone with vertical takeoff," using the abbreviation for North Korea's official name, the Democratic People’s Republic of Korea.

As Kim's trip draws to a close, his itinerary also highlights North Korea's interest in promoting collaboration with Russia across various sectors, including culture and education.

Kim, clad in a black suit, was seen strolling through the campus of the Far Eastern Federal University on Russky Island off Vladivostok on Sunday, according to videos released by Russian state media.

Kim later visited the Primorsky Aquarium, which is one of the most renowned tourist attractions in the Far East region. He reportedly watched captivating performances featuring beluga whales, dolphins, fur seals and walruses with Kozhemyako and Russian Natural Resources Minister Alexander Kozlov among others.

On Saturday night, Kim had the opportunity to delight in a performance of Pyotr Tchaikovsky's ballet "Sleeping Beauty" alongside local residents at the Primorsky stage of the Mariinsky Theater in Vladivostok.

This followed his meeting with Russian Defense Minister Sergei Shoigu on Saturday, during which they discussed practical matters aimed at enhancing their defense and security cooperation.

Kim’s trip to the port city of Vladivostok the previous day came at a juncture “where a new heyday of friendship, solidarity and cooperation is opening in the history of the development of relations between the DPRK and Russia," North Korean state media, including Korean Central News Agency, reported Sunday in a Korean-language dispatch.

Kim proceeded to the Knevichi Military Airfield on the outskirts of Vladivostok, where he was welcomed by Shoigu and other senior Russian military officials, upon his arrival in the city of Artyom by train on Saturday morning, North Korean state media reported Sunday in a Korean-language dispatch.

Kim was accompanied by a group of key military officials, including Defense Minister Kang Sun-nam, Adm. Kim Myong-sik, commander of the North Korean Navy and Gen. Kim Kwang-hyok, who commands the Air Force.

"Comrade Kim Jong-un inspected the displayed military aircraft and learned about their combat capabilities and onboard weaponry while receiving explanations about tactical and technical specifications," read the North Korean state media report.

North Korean state media did not provide specific details about the types of warplanes that Kim observed at the airfield.

However, Shoigu showcased several significant aircraft including the Tu-95MS turboprop strategic bomber, as well as the Tu-160 and Tu-22M3 supersonic bombers, known for their capability to carry nuclear weapons, which are integral to Russia's nuclear air attack force, according to Russian media and video footage released by the Russian Defense Ministry.

Additionally, Shoigu presented Kim with Russia's Kinzhal hypersonic missiles, which are capable of carrying both nuclear and conventional warheads, mounted on the MiG-31I supersonic interceptor aircraft. It's noteworthy that Russia has deployed the Kinzhal missiles against Ukraine.

In the video, Kim was observed examining the Kinzhal missile. The scene parallels a prior event in November 2022 when South Korean Defense Minister Lee Jong-sup and US Defense Secretary Lloyd Austin received briefings on the capabilities and operational procedures of B-52 and B-1B bombers at Andrews Air Force Base in Maryland as part of their annual Security Consultative Meeting.

South Korean Vice Defense Minister Shin Beom-chul had visited the same base on the occasion of the high-level meeting of the South Korea-US level Extended Deterrence Strategy and Consultation Group, or EDSCG, in September 2022. Shin had also similarly inspected the section of a B-52 strategic bomber where nuclear warheads can be loaded.

Kim and Shoigu subsequently journeyed to Vladivostok, where they conducted an inspection of the Pacific Fleet frigate, the Marshal Shaposhnikov.

"The Pacific Fleet stands as a formidable naval fortress in Russia's national defense, ensuring dependable safeguarding of Russia's territorial waters and national interests by maintaining a constant military readiness for military operations," North Korean state media reported.

The Pacific Fleet is further composed of "a diverse array of surface ships, various submarine types, including strategic nuclear submarines, an aviation corps and cutting-edge strategic weaponry and equipment."

Shoigu's presentation of advanced weaponry at the airfield and the Pacific Fleet frigate held particular significance, especially considering that Kim had previously showcased North Korea's newly developed weapons, including intercontinental ballistic missiles or ICBM, during Shoigu's visit to Pyongyang on the occasion of the July 27 armistice day marking the end of the Korean War.

In Vladivostok, Kim and Shoigu engaged in another face-to-face meeting, marking yet another encounter in less than two months.

"Comrade Kim Jong-un and Defense Minister Shoigu share views on the regional and international military-political situation," North Korean state media report.

"Both exchanged constructive opinions on practical matters to further strengthen strategic and tactical cooperation and enhance support and mutual exchanges between their respective armed forces and in the realms of defense and security between the two countries."

Cheong Seong-chang, director of the Department of Reunification Strategic Studies at the Sejong Institute, said the relations between North Korea and Russia "would evolve into a comprehensive strategic alliance that transcends the Cold War-era alliance."

"The Soviet Union provided support to North Korea in conventional weaponry during the era of the Cold War, but they did not cooperate in the field of strategic weapons," Cheong said.

"With Kim Jong-un's visit to Russia as momentum, it is anticipated that Russia will actively cooperate in assisting North Korea in developing reconnaissance satellites and nuclear-powered submarines the country needs and modernizing its navy and air forces."

Kim's visit to the Far East region has predominantly centered on activities symbolizing the mutual commitment of Russia and North Korea to strengthen military cooperation.

Kim and Putin met at the Vostochny Cosmodrome space launch center on Wednesday, which was later widely dismissed as the "pariah summit."

The Kremlin has confirmed its readiness to assist North Korea in improving its satellite technologies, which has drawn additional criticism and raised concerns about the dual-use nature of such advancements. It's important to note that satellite technologies can have applications that intersect with ICBM technologies.

On Friday, Kim visited aviation facilities in the far eastern city of Komsomolsk-on-Amur: the Yuri Gagarin Aviation Plant and the Yakovlev plant. Both of these facilities are units of the United Aircraft Corp., which is currently under Western sanctions.

Kim toured the jet production facilities and assembly workshops for various aircraft, encompassing the Sukhoi Su-35 fighter jets, the Su-57 fifth-generation fighter jet, and the new Sukhoi Superjet-100 passenger aircraft.

Clear indications of enhanced military ties have continued to emerge despite Seoul and Washington urging North Korea and Russia to cease such illegal cooperation.

The treaty allies have underscored that both Russia and North Korea appear to be seeking reciprocal exchanges, involving North Korea's provision of weapons and Russia's transfer of advanced weapon technologies.

At the EDSCG meeting, Seoul and Washington on Friday discussed ways to counter Russia's facilitation of North Korea's illicit nuclear and ballistic missile programs and North Korea's assistance to Russia’s illegal war effort in Ukraine.

"Military cooperation between Russia and North Korea constitutes a serious breach of UN Security Council resolutions," South Korean Vice Foreign Minister Chang Ho-jin emphasized after the meeting.

"South Korea and the US will make sure to impose clear consequences for such actions, all while maintaining close information sharing."