Chancellor Olaf Scholz reiterated Sunday that Germany will not send fighter jets to Ukraine, as Kyiv steps up calls for more advanced weapons from the West to help repel Russia's invasion.

Scholz only just agreed on Wednesday to send 14 Leopard 2 tanks to Ukraine and to allow other European countries to send theirs, after weeks of intense debate and mounting pressure from allies.

"I can only advise against entering into a constant bidding war when it comes to weapons systems," Scholz said in an interview with the Tagesspiegel newspaper.

"If, as soon as a decision (on tanks) has been made, the next debate starts in Germany, that doesn't come across as serious and undermines citizens' confidence in government decisions."

Scholz's decision to green-light the tanks was accompanied by a US announcement that it would send 31 of its Abrams tanks.

Ukrainian President Volodymyr Zelensky thanked Berlin and Washington for the move, seen as a breakthrough in efforts to support the war-torn country.

But Zelensky immediately stressed that Ukraine needed more heavy weapons from NATO allies to fend off Russian troops -- including fighter jets and long-range missiles.

Scholz in the interview warned against raising "the risk of escalation", with Moscow already sharply condemning the tank pledges.

"There is no war between NATO and Russia. We will not allow such an escalation," he said.

The chancellor added that it was "necessary" to continue speaking with Russian President Vladimir Putin. The last phone call between the leaders was in early December.

"I will talk to Putin by phone again," Scholz said.

"But of course it's also clear that as long as Russia continues to wage war with unabated aggression, the current situation will not change."

NATO asks South Korea to 'step up' military support for Ukraine

NATO Secretary General Jens Stoltenberg asked South Korea on Monday to "step up" military support for Ukraine, suggesting it reconsider its policy of not exporting weapons to countries in conflict.

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.

Russian shelling of Kherson leaves at least three dead

Russian shelling of Ukraine's southern city of Kherson left at least three people dead on Sunday, President Volodymyr Zelensky said, while a strike on Kharkiv killed one person, according to the regional governor.

"Today, the Russian army has been shelling Kherson atrociously all day," Zelensky said in his evening address.

"Two women, nurses, were wounded in the hospital. As of now, there are reports of six wounded and three dead."

The front in southern Ukraine has been considerably quieter recently than in the east, with Moscow withdrawing from Kherson city in November last year.

But the key city and regional capital is still subject to frequent shelling.

In eastern Kharkiv, the second-largest city in Ukraine, the governor of the regional military administration said a Russian strike hit "a four-storey residential building".

"Three victims received minor injuries. Unfortunately, an elderly woman died ... The building was partially destroyed," Oleh Synehubov said on Telegram.

In the Zaporizhzhia region of southern Ukraine, where fighting intensified in recent days after several months of a stagnant front, Moscow-appointed officials said Kyiv struck a railway bridge, killing four people.

Ukraine on Sunday carried out an "attack from a HIMARS multiple rocket launcher on a railway bridge across the Molochnaya river", the Russian-installed head of the region, Yevgeny Balitsky, wrote on social media.

"Four people from the railways brigade were killed, five were injured," Balitsky added.

The bridge is in a village north of the Russian-controlled city of Melitopol and was undergoing repairs, according to Balitsky.

Russia claims to have annexed the Zaporizhzhia and Kherson regions along with two other Ukrainian regions in the east, but does not fully control these territories.