South Korea is expected to call in the top Russian envoy in Seoul on Friday to lodge a protest over a new treaty signed with North Korea calling for immediate military assistance if either is attacked.

First Vice Foreign Minister Kim Hong-kyun will call in Russian Ambassador Georgy Zinoviev later in the day to deliver Seoul's position on the "comprehensive strategic partnership" treaty inked between Moscow and Pyongyang at their leaders' summit Wednesday, diplomatic sources said.

Kim is expected to stress that Russia's military cooperation with the North is a clear violation of the UN Security Council resolutions banning activities that would help Pyongyang's arms buildup.

Kim is also expected to make it clear to Russia about the possibility of Seoul providing Ukraine with arms support in response to Moscow's signing of the treaty with the North, as announced the previous day by the National Security Council.

The treaty states that if either of the two gets invaded or put in a state of war, the other side will provide military and other assistance "with all means" at its disposal and "without delay."

Article 4 of the treaty is seen as warranting automatic military intervention in the event of an attack on either country, a provision that restores the Cold War-era alliance 28 years after their mutual defense treaty was scrapped in 1996.

South Korea's presidential office condemned the pact as a threat to national security in violation of UN Security Council resolutions. It warned of a negative impact on its relations with Moscow.

Chang Ho-jin, South Korea's top security adviser, said after holding a National Security Council meeting Thursday that Seoul will reconsider its stance on the arms supply to Ukraine. South Korea has so far maintained a policy of only providing nonlethal aid to Kyiv.

Russian President Vladimir Putin, who was on a state visit to Vietnam, warned that it would be a "very big mistake" if South Korea provided lethal weapons to Ukraine.

Attending a UN session in New York on Thursday, Foreign Minister Cho Tae-yul said it was "deplorable" that Russia violated the resolutions that it itself has agreed to adopt as a permanent UNSC member.

Cho called on the international community to stand united against "any direct or indirect" cooperation that would help the North's military buildup that constitutes a violation of UNSC resolutions. (Yonhap)