Chang Ho-jin, director of the presidential National Security Office, speaks in a briefing in Seoul Thursday. (Yonhap)

South Korea on Thursday said it would review its arms support to Ukraine, while expressing serious concern over a summit between the leaders of North Korea and Russia where the two agreed to provide military assistance if either country is under attack.

While unveiling plans to impose additional sanctions on North Korean and Russian individuals and institutions, National Security Adviser Chang Ho-jin told reporters in a briefing in Seoul that the office will "reconsider the issue of providing arms support to Ukraine."

The remark came after the office expressed grave concern and condemnation over the comprehensive strategic partnership treaty signed by North Korea and Russia on military and economic cooperation Wednesday. Seoul criticized it as "irresponsible rhetoric," noting that both countries, which initiated the 1950-53 Korean War and the Ukraine war, respectively, are promising military cooperation based on the assumption of a preemptive strike that has not occurred.

Any aid to North Korea's military buildup violates UN Security Council resolutions and is subject to international sanctions, the office said, stressing that as a UN Security Council member Russia is undermining security and harming its relations with South Korea by supporting North Korea.

"The government will firmly respond to any threats to its security in cooperation with the international community and will strengthen ROK-US extended deterrence and ROK-US-Japan security cooperation to neutralize North Korea's nuclear and missile threats," a government statement read by Chang said, referring to South Korea, which is formally the Republic of Korea.

On weapon support to Ukraine, the office elaborated that could be one of many options aimed at pressuring Russia.

"Regarding the review of arms support, I won't specifically comment on whether they are lethal weapons or not. Weapons are just one of many options," a senior presidential official said on condition of anonymity.

"There are various ways to classify them as lethal or nonlethal. Russia will find it increasingly interesting, and there's no need to give an answer in advance," he said. "Revealing it gradually will create pressure."

Ministry of Foreign Affairs spokesperson Lim Soo-suk speaks at a briefing at the Government Complex Seoul on Thursday. (Yonhap)

Earlier in the day, the Foreign Ministry in Seoul also pledged stern actions jointly with its allies and the international community against any provocations in the wake of Russian President Vladimir Putin's summit with North Korean leader Kim Jong-un on Wednesday.

"We express grave concern over Russia and North Korea signing the comprehensive strategic partnership pact and openly commenting on military technology cooperation, which is a clear violation of UN resolutions, despite a series of warnings from the international community, including ourselves," Lim Soo-suk, spokesperson for the Foreign Ministry in Seoul, said in a briefing Thursday.

"After shrewdly analyzing and assessing the outcome of (Putin's) visit to North Korea, we will take stern actions correspondingly, against any threat to our national security, together with the international community including our allies and friendly nations."

Also in response to the pact, Japanese Chief Cabinet Secretary Yoshimasa Hayashi expressed concern that Putin did not rule out military technology cooperation with North Korea after his summit with Kim on Wednesday, at a press briefing Thursday.

China's Foreign Ministry spokesperson Lin Jian on Thursday declined to comment on questions asking about China's position on the elevated ties between North Korea and Russia, saying it is a bilateral matter. The spokesperson also declined to comment on where China stands in the confrontation between the Russia-North Korea bloc and the bloc comprising South Korea, the US and Japan.

North Korean leader Kim Jong-un and Russian President Vladimir Putin tour the garden of the Kumsusan State Guest House in Pyongyang in an Aurus, a Russian vehicle given to Kim as a gift, according to the official Korea Central News Agency on Thursday. (KCNA via Yonhap)

The pact, released a day after Kim and Putin signed it Wednesday in Pyongyang, acknowledged that each country would provide "all available military and other assistance if the other faced armed aggression," effectively reviving the automatic military engagement clause in the 1961 treaty signed between North Korea and the Soviet Union.

The two countries will take joint action to strengthen defense capabilities, according to the pact released by the North's official Korean Central News Agency. Additionally, neither North Korea nor Russia would sign any treaty with a third country infringing on the interest of the counterpart of Wednesday's pact, and neither would allow a third country to infringe on security or sovereignty on their territory.

Also under the pact, Pyongyang and Moscow will cooperate on issues of mutual interest at the United Nations, on a range of technologies for space, "peaceful use" of nuclear energy and artificial intelligence and on the construction of a motorway bridge across the border of two countries along the Tumen River.

A source from the Unification Ministry said on condition of anonymity Thursday, "We see that the two countries boasted of an anti-West, anti-US coalition by elevating the two countries' ties with a major focus on military cooperation," when asked about the impression of the Kim-Putin talks Wednesday in Pyongyang just before the pact was announced.

"North Korea could garner support from Russia over its possible confrontation with the bloc comprising South Korea, the US and Japan, while Russia could assure North Korea's support in the ongoing war with Ukraine," the source added. "Russia also looks to break Western isolation and at the same time exert its influence on the Korean Peninsula."

When asked about the difference between the 1961 bilateral pact and the pact signed Wednesday, the official said that Wednesday's "automatic military intervention" clause in the pact is in accordance with the respective laws of both North Korea and Russia, as well as the UN Charter's Article 51 stipulating the "inherent right of individual or collective self-defense" in the event of an armed attack.

"We should find out why these are included," the Unification Ministry official said.