Stoltenberg is in Seoul on the first leg of his Asia trip, which will also take in Japan, as part of a drive to boost ties with the region's democratic allies in the face of the Ukraine conflict and growing competition from China.

He met top South Korean officials Sunday, and on Monday urged Seoul to do more to help Kyiv, saying there was an "urgent need for more ammunition".

He pointed to countries like Germany and Norway that had "long standing policies not to export weapons to countries in conflict" which they revised after Russian President Vladimir Putin invaded Ukraine in February last year.

"If we believe in freedom, democracy, if we don't want autocracy and totalitarian to win then they need weapons," he said, speaking at the Chey Institute in Seoul.

South Korea is an increasingly important arms exporter globally and has recently signed deals to sell hundreds of tanks to European countries, including NATO-member Poland.

But South Korean law bans the export of weapons to countries in active conflict, which Seoul has said makes it difficult to provide arms directly to Kyiv, although it has provided non-lethal and humanitarian assistance.

South Korea opened its first diplomatic mission to NATO last year.

Stoltenberg said it was unclear when the conflict in Ukraine would end, saying Putin was preparing for "more war" and actively acquiring weapons from countries including North Korea.

Pyongyang has denied sending weapons to Moscow, and said Sunday that the US would face a "really undesirable result" if it persisted in spreading the "self-made rumour".

"Trying to tarnish the image of (North Korea) by fabricating a non-existent thing is a grave provocation that can never be allowed and that cannot but trigger its reaction," said Kwon Jong Gun, director general of the North's Department of US Affairs.

He also called it "a foolish attempt to justify its offer of weapons to Ukraine".

Earlier this week, US President Joe Biden promised 31 Abrams tanks, one of the most powerful and sophisticated weapons in the US army, to help Kyiv fight off Moscow's invasion.