Russia moved to ratify a key defence pact with North Korea on Thursday, while South Korea warned it wound not "sit idle" if Pyongyang deployed thousands of troops to help Moscow fight Ukraine.
Seoul's spy agency says thousands of North Korean soldiers are currently training in Russia and are likely to deploy to the front lines in Ukraine soon, with thousands more to be sent by December.
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Lawmakers in Russia's lower house of parliament voted unanimously on Thursday to ratify a treaty with North Korea that provides for "mutual assistance" if either party faces aggression.
The accord will be now sent to the upper house, the Federation Council, for its approval.
Both houses of parliament act as rubber stamps for the Kremlin.
The West believes North Korea is already giving Moscow weapons to use in its Ukraine offensive.
"South Korea won't sit idle over this," South Korean President Yoon Suk Yeol said of Pyongyang's reported troop deployments, after talks with visiting Polish President Andrzej Duda.
The two countries agreed North Korea's deployment was "a provocation that threatens global security beyond the Korean Peninsula and Europe", he added.
South Korea, one of the world's top 10 weapons exporters, has long resisted calls from its allies, including Washington, to supply Kyiv with weapons.
But it has hinted it could review this policy in light of North Korea's actions and Yoon said Thursday that Seoul would "take necessary actions in cooperation with the international community" to respond.
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Pyongyang and Moscow have drawn closer since Russia launched its 2022 military offensive on Ukraine.
North Korean leader Kim Jong Un has praised Russian President Vladimir Putin as his country's "dearest friend".
Moscow on Wednesday refused to confirm or deny reports of North Korean soldiers being sent to Russia, telling reporters to "ask Pyongyang" where its troops were.
- Tanks to Poland -
Seoul has already sold billions of dollars of tanks, howitzers, attack aircraft and rocket launchers to Poland, a key ally of Kyiv's.
In June, South Korea agreed to transfer the knowledge needed to build K2 tanks to Poland, which experts have said could be a key step towards production inside the territory of Ukraine.
The two countries will "actively support the successful progress of the Korea-Poland defence cooperation", Yoon said.
This will include signing a deal on a second contract for South Korean K2 tanks by the end of the year, he added.
They also announced they would "strengthen joint efforts for the restoration of peace and reconstruction in Ukraine" and "continue to expand support for the Ukrainian people and work closely with Poland in the process".
President Duda's four-day visit to South Korea will end on Friday, with a stop to Hyundai Rotem, producers of the K2 tanks, and to Hanwha Aerospace, South Korea's largest defence contractor.
Hanwha Aerospace has signed a $1.64-billion deal with Poland to supply rocket artillery units.
A South Korean official from the president's office told reporters on Tuesday that Seoul would "support (Ukraine) through defensive weaponry, and if things get out of line, we could consider sending offensive weapons".
Prior to Yoon and Duda's meeting, a North Korean balloon carrying trash landed on Seoul's presidential compound.
Local media reported it contained propaganda leaflets ridiculing the South Korean president and his wife.
Photographs released by local media showed a leaflet featuring South Korean first lady Kim Keon Hee's picture alongside the phrase: "Queen Kim Keon Hee, a figure who rivals Marie Antoinette, the epitome of luxury and indulgence."
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