HANOI — 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.

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

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 cease" military cooperation with Pyongyang.

While South Korea has given humanitarian aid and military equipment to Ukraine, it has so far refused to provide lethal weapons as it has an official policy not to arm countries at war.

Some in Ukraine have been hoping that the deepening military collaboration between Moscow and Pyongyang would cause Seoul to rethink its approach. Analysts had earlier said that Kyiv would use Putin's visit to Pyongyang to up the pressure.

During the visit, Kim had also pledged "full support" for Russia's invasion of Ukraine. There is growing evidence that Russia has already been deploying North Korean missiles in Ukraine.

Early on Friday, the US National Security Council's spokesman John Kirby weighed in on the Russian-North Korean agreement, saying it should "be of concern to any country that cares about maintaining peace and stability" in the region.

He added that the agreement was "no surprise", saying that the US had been warning about the two countries' "burgeoning defense relationship" for many months.

Tokyo said it was “seriously concerned that President Putin did not rule out military technology cooperation with North Korea”, Japan's government spokesman Yoshimasa Hayashi said, adding that the agreement was "unacceptable".

Analysts have said that the treaty could have significant implications for the world as well as the region. Besides the possibility of North Korea openly arming Russia, it could also potentially see Russia intervening in any fresh conflict on the Korean peninsula.

The two Koreas are still technically at war and maintain a heavily guarded border, where tensions have worsened in recent weeks.

In a separate incident on Thursday, North Korean troops "briefly crossed" the border and retreated after the South fired warning shots, Seoul authorities said on Friday.

This marks the third such incident in less than three weeks. South Korea's Joint Chiefs of Staff had said the two earlier cases — on 9 June and 18 June — appeared to be unintentional. — BBC