Theresa May and South Korea's new president have agreed the need for a "robust response" to Pyongyang's violations of United Nations resolutions.
The Prime Minister called Moon Jae-in to congratulate him on his election victory and to discuss the crisis around North Korea's nuclear weapons test programme.
A Downing Street spokesman said the two leaders vowed to remain "close partners" on issues including defence and security.
The spokesman said: "The Prime Minister called the South Korean President Moon Jae-in earlier this morning to congratulate him on his recent electoral success.
"They agreed the UK and Republic of Korea would remain close partners on a wide range of issues, including trade, defence and security. They agreed the bilateral relationship would continue to go from strength to strength.
"They agreed it would be important for the international community to continue to pursue a robust response to North Korean violations of UN Security Council resolutions and other international obligations."