By Hyonhee Shin

SEOUL, July 10 (Reuters) - South Korean President Yoon Suk Yeol was set to depart on Monday for a summit with NATO leaders, seeking deeper international security cooperation amid rising North Korean threats and tension over China.

Yoon's attendance at the annual NATO gathering that begins in Lithuania on Tuesday, alongside the leaders of Japan, Australia and New Zealand, will be followed by a three-day visit to Poland starting on Wednesday, his office said.

Yoon has been pushing for greater security ties with Europe and other U.S. allies to deter North Korea's nuclear and missile threats, while raising its contributions to global challenges, including Russia's invasion in Ukraine and Sino-U.S. rivalry.

Last year, he attended the NATO summit for the first time as a South Korean leader, saying new conflicts and competition posed threats to universal values.

Yoon this year is looking to voice stronger messages against North Korea and step up cooperation on security, supply chains and the Ukraine war, a presidential official said.

He will hold bilateral talks with several European and Asia Pacific leaders on the sidelines of the summit, and separately adopt a new document with NATO to bolster bilateral cooperation in 11 areas, including non-proliferation and cybersecurity.

"He would strengthen cooperation with NATO on North Korea's escalating nuclear and missile threats, and send a united warning that the international community will not tolerate North Korea's illegal activities," the official told reporters.

A U.S. ally and rising arms exporter, South Korea may face renewed pressure to provide weapons to Ukraine, which Yoon's administration has resisted, wary of Russian influence over North Korea. There had also been speculation in media that Yoon might visit Ukraine as part of the trip.

The official said Yoon has no plans to travel to Ukraine, but would highlight Seoul's commitment to restoring peace in Ukraine and explore aid packages with other countries.

In a written interview with The Associated Press published on Monday, Yoon said supplies to Ukraine, including de-mining equipment and ambulances, "are in the works" and vowed support for post-war reconstruction.

In Poland, which Yoon will visit for the first time as president, both sides are expected to discuss ways to boost economic and strategic ties, including in arms trade and nuclear energy.

Poland has emerged a key destination of South Korean defence exports as it seeks to beef up its military in the face of Russia's aggression against Ukraine. Last year, two South Korean companies signed a $5.76 billion contract with Poland to export tanks and howitzers, as part of South Korea's biggest ever arms deal.