North Korea's leader Kim Jong Un (center R) and Russia's President Vladimir Putin (center L) visiting the Vostochny Cosmodrome in Russia's Amur region. [AFP]

South Korea has expressed regret that Russia and North Korea discussed military cooperation during their leaders' meeting, as Seoul is closely monitoring the possibility of joint military drills between Pyongyang and Moscow.

Officials said South Korean President Yoon Suk Yeol will use his address next week at the United Nations General Assembly to stress grave concern over the military cooperation between North Korea and Russia.

"Despite repeated warnings from the international community, I am deeply concerned and regretful that the North Korea-Russia summit has discussed the issue of military cooperation, including satellite development," South Korea's foreign ministry spokesperson Lim Soo-suk told reporters on Thursday.

Seoul is seeking a meeting with Russian diplomats, a day after Russian President Vladimir Putin and North Korean leader Kim Jong Un held talks at Russia's most modern spaceport, the Vostochny Cosmodrome.

Putin has accepted an invitation from Kim to visit North Korea, according to Pyongyang's state media.

"We continue to communicate necessarily between South Korea and Russia," said Lim, who warned a Russia-North Korea military cooperation would bring a "negative impact" to bilateral ties between Moscow and Seoul.

As President Yoon heads to New York next week for the U.N. General Assembly, South Korean officials have renewed warnings. They said Russia's science and technology cooperation with North Korea that contributes to its development of nuclear weapons and missiles, including satellites using ballistic missile technology, is prohibited by UNSC resolutions.

South Korea's National Security Council members met Thursday afternoon to discuss North Korean leader Kim's visit to Russia.

South Korea is "taking the situation very seriously," regarding "the fact that various military cooperation, including the development of intercontinental ballistic missiles, was discussed" during Kim's meeting with Putin, according to South Korea's National Security Council.

"We will discuss the issue of North Korea-Russia military cooperation in consultation with the U.S., Japan and the international community."

Thursday, Kim is expected to visit military and civilian aviation factories in the Russian city of Komsomolsk-on-Amur, according to Russia's state media.

South Korea's defense ministry said it continues to closely monitor Kim's schedule in Russia's Far East, and potential joint naval exercises between Russia and North Korea said to be proposed during Russian Defense Minister Sergei Shoigu's visit to Pyongyang in July.

"The two sides did not issue a joint statement, but the personnel Kim brought to Russia, and where he and Putin met, were quite telling. The composition of Kim's delegation suggests North Korea may send munitions to Russia in exchange for military technology," said Leif-Eric Easley, a professor at Ewha University in Seoul.

"If Putin and Kim just wanted to do an arms deal, they didn't need to meet in person and could have tried to keep their additional sanctions violations secret while continuing to deny them," Easley said. "In addition to supporting each other's illegal trade, Moscow and Pyongyang are showcasing their cooperation to score points in domestic politics."

North Korea did not name the members of its delegation, but several top military commanders, arms industry officials and diplomats appear to be accompanying Kim in photos released by North Korean state media, including North Korean Defense Minister Kang Sun Nam and Marshal Pak Jong Chon, new head of the party's military political leadership.