Russian President Vladimir Putin warned South Korea against supplying weapons to Ukraine, saying it would be a “very big mistake.” Putin made the comments at a press conference during a state visit to Vietnam on Thursday. Kremlin Pool Photo by Gavriil Grigorov/Sputnik/EPA-EFE

“As for the supply of lethal weapons to the combat zone in Ukraine, this would be a very big mistake,” Putin said. “I hope this doesn’t happen. If this happens, then we, too, will make appropriate decisions that are unlikely to please the current leadership of South Korea.”

The Russian president added that Moscow is not ruling out supplying weapons to North Korea to counter “acts of aggression” by Washington and Seoul.

“I said, including in Pyongyang, that we then reserve the right to supply weapons to other regions of the world,” Putin said. “Keeping in mind our agreements with the Democratic People’s Republic of Korea, I’m not ruling that out.”

The remarks came after South Korea’s presidential office said Thursday that it would consider sending lethal arms to Ukraine in the wake of a treaty signed by Russia and North Korea that involves a pledge of mutual military aid.

Seoul has provided humanitarian aid to Ukraine and sold weapons to neighboring Poland, but has stuck to a policy of not sending deadly weapons directly to any country during a conflict.

South Korea “expresses grave concern and condemns the signing of the comprehensive strategic partnership agreement between North Korea and Russia” National Security Advisor Chang Ho-jin said at a press briefing.

Chang called the closer ties between North Korea and Russia “troubling” and said any military assistance to the North was a violation of U.N. Security Council resolutions.

“As for arms support to Ukraine, the issue will be reexamined,” he said.

Putin and North Korean leader Kim Jong Un signed the treaty Wednesday night during the Russian leader’s first visit to the North since 2000.

Article 4 of the 23-point document calls for each country to “provide military and other assistance without delay by all means at its disposal” if the other is attacked and goes to war.

South Korea’s Foreign Ministry summoned Russia’s top envoy in Seoul on Friday to express concern over the pact.

First Vice Foreign Minister Kim Hong-kyun called in Russian Ambassador Georgy Zinoviev and “conveyed our government’s stern position on the decision to strengthen mutual military and economic cooperation,” the ministry said in a statement.

Kim “strongly urged Russia to immediately cease military cooperation with North Korea and comply with Security Council resolutions.”

Moscow and Pyongyang have bolstered ties following a state visit by Kim in September, with North Korea supplying munitions to Russia for its war in Ukraine.

U.S. State Department spokesman Matt Miller said earlier this week that the North has transferred “dozens of ballistic missiles and over 11,000 containers of munitions to aid Russia’s war effort.”

Washington and its allies claim that Russia is likely providing fuel, raw materials and advanced space and weapons technology to North Korea in exchange.