The comments by a senior presidential official came hours after North Korea's state media released the details of the agreement reached between its leader Kim Jong Un and Russian President Vladimir Putin during their summit on Wednesday in Pyongyang.
The North's official Korean Central News Agency said the deal requires both countries to use all available means to provide immediate military assistance in the event of war.
The office of South Korean President Yoon Suk Yeol issued a statement condemning the agreement, calling it a threat to the South's security and a violation of U.N. Security Council resolutions, and warned that it would have negative consequences on Seoul's relations with Moscow.
The presidential official, who spoke on condition of anonymity during a background briefing, according to office rules, said Seoul in response will reconsider the issue of providing arms to Ukraine to help the country fight off Russia's invasion.
South Korea, a growing arms exporter with a well-equipped military backed by the United States, has provided humanitarian aid and other support to Ukraine while joining U.S.-led economic sanctions against Moscow. But it has not directly provided arms to Ukraine, citing a long-standing policy of not supplying weapons to countries actively engaged in conflict.
Both Kim and Putin described their deal as a major upgrade of bilateral relations, covering security, trade, investment, cultural and humanitarian ties. Outside observers said it could mark the strongest connection between Moscow and Pyongyang since the end of the Cold War.
KCNA said Article 4 of the agreement states that if one of the countries gets invaded and is pushed into a state of war, the other must deploy "all means at its disposal without delay" to provide "military and other assistance." But it also says that such actions must be in accordance with the laws of both countries and Article 51 of the United Nations Charter, which recognizes a U.N. member state's right to self-defense.
The summit between Kim and Putin came as the U.S. and its allies expressed growing concern over a possible arms arrangement in which Pyongyang provides Moscow with badly needed munitions for its war in Ukraine, in exchange for economic assistance and technology transfers that could enhance the threat posed by Kim's nuclear weapons and missile program.
Following their summit, Kim said the two countries had a "fiery friendship," and that the deal was their "strongest-ever treaty," putting the relationship at the level of an alliance. He vowed full support for Russia's war in Ukraine. Putin called it a "breakthrough document" reflecting shared desires to move relations to a higher level.
North Korea and the former Soviet Union signed a treaty in 1961, which experts say necessitated Moscow's military intervention if the North came under attack. The deal was discarded after the collapse of the USSR, replaced by one in 2000 that offered weaker security assurances.
A full day after the summit, South Korean officials said they were still interpreting the results, including what Russia's response might be if the North comes under attack. Analysts were mixed on whether the agreement obligates Russia to an automatic military invention on behalf of the North in war situations or was carefully worded enough to avoid such a commitment. It also wasn't immediately clear why the article invokes the U.N. charter.
"We are currently reviewing the specifics of the treaty signed between Russia and North Korea during President Putin's visit to North Korea. We will announce our government's position after we are done," Lim Soosuk, South Korea's Foreign Ministry spokesperson, said during a briefing.
Still, Lim expressed regret that Moscow and Pyongyang signed the agreement while openly talking about military and technology cooperation that would be in violation of U.N. Security Council resolutions.
"Based on our close analysis and assessment of the results of (Putin's) visit, including the comprehensive strategic partnership treaty signed between Russia and North Korea, we will work with the international community, including our allies and friends, to take correspondingly stern and decisive measures to any actions that threaten our security," Lim said.
The deal was made as Putin visited North Korea for the first time in 24 years, a visit that showcased their personal and geopolitical ties with Kim hugging Putin twice at the airport, their motorcade rolling past giant Russian flags and Putin portraits, and a welcoming ceremony at Pyongyang's main square attended by what appeared to be tens of thousands of spectators.
According to KCNA, the agreement also states that Pyongyang and Moscow must not enter into agreements with third parties if they infringe on the "core interests" of another and must not participate in actions that threaten those interests.
KCNA said the agreements require the countries to take steps to prepare joint measures for the purpose of strengthening their defense capabilities to prevent war and protect regional and global peace and security. The agency didn't specify what those steps are, or whether they would include combined military training and other cooperation.
The agreement also calls for the countries to actively cooperate in efforts to establish a "just and multipolar new world order," KCNA said, underscoring how the countries are aligning in face of their separate, escalating confrontations with the Untied States.
Kim in recent months has made Russia his priority as he pushes a foreign policy aimed at expanding relations with countries confronting Washington, embracing the idea of a "new Cold War" and trying to display a united front in Putin's broader conflicts with the West.
Tensions on the Korean Peninsula are at their highest point in years, with the pace of both Kim's weapons tests and combined military exercises involving the U.S., South Korea and Japan intensifying in a tit-for-tat cycle.
The Koreas also have engaged in Cold War-style psychological warfare that involved North Korea dropping tons of trash on the South with balloons, and the South broadcasting anti-North Korean propaganda with its loudspeakers.