Russian President Vladimir Putin has warned South Korea against providing weapons to Ukraine, saying that such provision will ruin relations between Seoul and Moscow.

According to Reuters and other media, Putin issued the warning on Thursday while speaking at a meeting of the Valdai Discussion Club, a conference of international foreign policy experts, in Moscow.

Russia’s state-owned Sputnik news agency reported that the president told the conference that he was aware of South Korea’s intention to supply arms and ammunition to Ukraine.

The very unusual warning by Putin to South Korea followed an claim that North Korea and the U.S. had almost reached an agreement on the North’s nuclear program, but Washington reversed course and slapped sanctions on Pyongyang.

South Korea has thus far sent Ukraine medical supplies, humanitarian aid and military supplies such as bulletproof helmets, tents and blankets, but has not offered any weapons under the principle that it cannot provide lethal weapons.