Vladimir Putin has warned South Korea it would be making "a big mistake" if it arms Ukraine in the war against Russia.

His comments come after Seoul said it was considering such a possibility, in response to Russia and North Korea's new pact to help each other in the event of "aggression" against either country, the BBC reported.

Moscow "will... [make] decisions which are unlikely to please the current leadership of South Korea" if Seoul decides to supply arms to Kyiv, Putin said.

The Russian leader was speaking in Vietnam, shortly after a lavish visit to Pyongyang where he signed a mutual defense agreement with North Korean leader Kim Jong Un.

Putin also warned that Moscow is willing to arm Pyongyang if the US and its allies continue supplying Ukraine with weapons.

"Those who supply these weapons believe that they are not at war with us. I said, including in Pyongyang, that we then reserve the right to supply weapons to other regions of the world," Putin said.

Seoul had earlier condemned the Russian-North Korean agreement as a threat to its national security, and national security adviser Chang Ho-jin had said his country planned to "reconsider the issue of arms support to Ukraine".

Following Putin's remarks, South Korea's presidential office said on Friday it would consider "various options" in supplying arms to Ukraine and its stance will "depend on how Russia approaches this issue".

It also summoned the Russian ambassador Georgy Zinoviev to protest the pact, demanding that Moscow "immediately ceases" military cooperation with Pyongyang.