South Korean President Lee Jae Myung on Friday threatened unspecified measures against Ukraine unless Kyiv acknowledged what Seoul says was a confidentiality agreement covering the transfer of two captured North Korean soldiers and apologises.

“If the refusal to acknowledge the facts and issue a public apology continues, we will take additional measures,” Lee said in a post on X.

Lee also expressed “grave regret” over what he described as Ukrainian remarks about an imminent military clash between North and South Korea, saying they amounted to encouraging war on the Korean Peninsula.

Seoul and Kyiv have been locked in a diplomatic dispute since Ukrainian President Volodymyr Zelenskiy announced at the United Nations General Assembly last month that Ukraine had transferred two North Korean prisoners of war captured while fighting for Russia to South Korea.

Seoul subsequently accused Kyiv of violating an agreement to keep the transfer confidential in order to protect the soldiers, while Ukrainian officials said no such agreement existed.

Ukraine was seeking to mend ties with South Korea, Ukrainian media reported on Thursday, quoting Foreign Minister Andrii Sybiha.

“I would not like to call this an incident. Let’s say, it’s a diplomatic misunderstanding,” Sybiha told journalists at an event, according to a report on news website nv.ua and other media.

The dispute comes at a sensitive time for Lee’s administration, which has sought to reduce tensions with North Korea and is also dealing with political fallout from a recent landmine incident in the Demilitarized Zone.

Cho Han-bum, a senior fellow at the Korea Institute for National Unification in Seoul, said any additional measures would most likely involve support South Korea already provides to Ukraine, including humanitarian, financial and reconstruction assistance.

“Ukraine has more to lose than South Korea because it is receiving support from Seoul,” Cho said.

South Korea has provided humanitarian and financial assistance to Ukraine since Russia’s 2022 invasion and in July pledged an additional $100 million in non-lethal support on the sidelines of the Nato summit.

“However, neither side benefits from a prolonged confrontation,” Cho said, noting that South Korea relies on Ukraine for information related to North Korea’s military activities.