Seoul and Washington will retaliate harshly if North Korea were to resort to the use of nuclear weapons, South Korean President Yoon Suk Yeol has warned.

The US and South Korea have “reaffirmed that any nuclear attack by North Korea will be met with a swift, overwhelming and decisive response,” Yoon said in an interview with AP on Sunday.  

A decision by the neighboring country to deploy nuclear weapons would “bring about the end of the regime” in Pyongyang, he said.  

North Korea, which has carried out a series of ballistic missile tests this year, has warned many times that it won’t hesitate to use nuclear weapons in order to defend itself. The country’s leader, Kim Jong-un, said earlier this year that Pyongyang’s nuclear strategic forces “will make perfect preparations for carrying out their important mission anytime” if Seoul and Washington don’t stop showing “open hostility” towards Pyongyang by staging large-scale military drills in and around the Korean Peninsula.

Yoon’s interview came out on the final day of Kim’s visit to Russia, a trip that caused concern in South Korea and the West. The North Korean leader had been in the country since Tuesday, holding talks with Russian President Vladimir Putin and Defense Minister Sergey Shoigu. He also inspected the Vostochny Cosmodrome and aviation plants in Russia’s Far East, and was shown various military hardware, including nuclear-capable strategic bombers.

North Korean outlet Voice of Korea said that during their talks on Saturday Kim and Shoigu discussed ways of “further strengthening… cooperation and mutual exchange between the armed forces of the two countries both in the field of defense and state security.”