President Yoon Suk Yeol says military cooperation between North Korea and Russia is “illegal” and “unjust” as it violates UN Security Council resolutions and other international sanctions.

Yoon made the remark in written responses to questions from the Associated Press on Sunday, a day before he departed for New York to attend the UN General Assembly, as he stressed that the international community “will unite more tightly in response to such a move.”

The South Korean leader is expected to issue such a warning when he delivers a keynote speech at the UN General Assembly on Wednesday.

The report also quoted Yoon as saying that Seoul and Washington have reaffirmed that any nuclear attack by Pyongyang “will be met with a swift, overwhelming and decisive response that will bring about the end of the regime.”

He added that the South Korea-U.S. extended deterrence “will develop into a joint system in which both countries discuss, decide and act together.”

On concerns that efforts to boost security cooperation among Seoul, Washington and Tokyo could hurt South Korea’s relations with China, Yoon said “the trilateral cooperation harbors no intention of marginalizing any particular nation or establishing an exclusive coalition.”