Russian President Vladimir Putin has warned it would be a "very big mistake" if South Korea provides lethal weapons to Ukraine after Seoul suggested such a possibility in response to Russia's commitment to military cooperation with North Korea.

The South Korean government said Thursday it will reconsider its stance on its arms supply to Ukraine after North Korea and Russia signed a treaty that involves military assistance without delay if one of them is attacked.

"As for the supply of lethal weapons to the combat zone in Ukraine, it would be a very big mistake. I hope it will not happen. If it does, then we too will then make the respective decisions, which South Korea's current leadership is unlikely to be pleased with," Putin told a press conference during his visit to Vietnam, according to Russia's news agency Sputnik.

South Korea's assistance to Ukraine has included military supplies, such as first-aid kits, medicine, portable mine detectors and protective suits, but the country has maintained a policy of not providing lethal aid.

Putin and the North's leader have agreed to offer military assistance "without delay" if either is attacked under the comprehensive strategic partnership treaty signed after this week's summit in Pyongyang.

Article 4 of the 23-point treaty could be seen as warranting automatic military intervention in the event of an attack on either country. That would amount to the restoration of a Cold War-era alliance for the first time in 28 years since a mutual defense treaty was scrapped in 1996. (Yonhap)