South Korean President Yoon Suk-yeol met with Ukrainian President Volodymyr Zelenskyy in an unannounced visit to Ukraine on Saturday, a gesture of support for Ukraine in its conflict with Russia while demonstrating South Korea's cooperation with NATO.

The two leaders discussed "the implementation of the Peace Formula and the preparation of the Global Peace Summit, food and energy security and economic cooperation," Zelenskyy tweeted.

Yoon's office said he traveled to Ukraine with his wife, Kim Keon Hee, following trips to Lithuania for a NATO summit and to Poland. It's his first such visit since the Russia-Ukraine conflict erupted almost 17 months ago.

A presidential official told Yonhap reporters that Yoon's visit came at Zelenskyy's invitation, and followed internal deliberations over security and safety, and the need for a visit before Yoon made the final decision.

The official said South Korea understood the invitation as a reflection of Ukraine's expectations for South Korea, and South Korea is "preparing comprehensive and concrete measures for assistance and cooperation between Ukraine."

He added that the upcoming summit is necessary for detailed discussions on cooperation during the crisis and in the post-conflict reconstruction effort.

South Korea has assisted Ukraine with humanitarian and financial support, such as medicines, portable mine detectors, protective suits, computers and generators, but the country has denied any request for lethal weapons in line with its long-standing policy of not supplying arms to countries actively engaged in conflict.

In January, during a visit to South Korea, NATO Secretary-General Jens Stoltenberg called for the country to provide direct military support to Ukraine, saying Kyiv was in urgent need of weapons. Two months later, news reports on a set of leaked Pentagon documents alleged the presidential office grappled with a U.S. request for ammunition for Ukraine.

Controversy arose in April after Yoon said in an interview with Reuters that South Korea could provide aid beyond humanitarian or financial support if Ukraine comes under a large-scale attack against civilians, signalling a shift in Seoul's policy of not providing lethal weapons to Ukraine.

Opposition leader Lee Jae-myung urged Yoon on Wednesday to reconsider his remarks suggesting the conditional possibility of providing non-humanitarian aid to Ukraine, saying the move would seriously hurt the national interest.

"Offering military support to a region in conflict is an act that harms the national interest and we should never do it," Lee told reporters after a party event at the National Assembly. "In diplomacy, nothing is more important than the lives and safety of our people and the national interest."

No previous governments had a foreign policy of creating an adversary, he said.

(With input from agencies)