Major weapons exporter South Korea will "reconsider" a longstanding policy that bars it from supplying arms directly to Ukraine, a presidential official said Thursday, after North Korea signed a mutual defence agreement with Russia.

"(We are) planning to reconsider the issue of providing weapons support to Ukraine," a presidential official told reporters hours after Russia's President Vladimir Putin left Pyongyang.

Putin signed a "breakthrough" new deal with North Korean leader Kim Jong Un during his state visit.

Seoul also expressed its "grave concern" over the agreement in which the two heavily sanctioned countries agreed to strengthen their military and economic cooperation, including immediate military assistance if either faced armed aggression. 

"Any cooperation that directly or indirectly helps strengthen North Korea's military capabilities is a violation of the UN Security Council resolutions," national security adviser Chang Ho-jin told reporters. 

"Russia's own violation of the resolution and support for North Korea will inevitably have a negative impact on the South Korea-Russia relationship," Chang said.

Western powers have stepped up sanctions aimed at constraining Russia's war in Ukraine.

Seoul has a longstanding policy that bars it from selling weapons into active conflict zones, which it has stuck to despite calls from Washington and Kyiv to reconsider.

It has, however, sold tanks and howitzers to Poland.