North Korea's foreign minister said on Saturday he was increasingly alarmed by US attitudes towards the isolated state but said it remained firm in its determination to implement the nuclear deal it reached with Washington in June.

The United States and North Korea pledged to end the North's nuclear programme and establish peace on the Korean peninsula in the landmark June agreement reached by their leaders in Singapore.

"The DPRK stands firm in its determination and commitment for implementing the DPRK-US joint statement in a responsible and good-faith manner," said Ri Yong Ho, referring to his country by its official name, the Democratic People's Republic of Korea.

"What is alarming however is the insistent moves manifested within the US to go back to the old, far from its leader's intention."

US Secretary of State Mike Pompeo pressed Southeast Asian nations at regional meetings in Singapore this week to maintain sanctions on North Korea but said he was optimistic that the deal to end the North's nuclear programme would be implemented.

"We have initiated goodwill measures of, inter alia, a moratorium on nuclear tests and rocket launch tests and dismantling of nuclear test ground," Ri said in a statement delivered to the Association of Southeast Asian Nations (ASEAN) forum.

"However, the United States, instead of responding to these measures, is raising its voice louder for maintaining the sanctions against the DPRK and showing the attitude to retreat even from declaring the end of the war, a very basic and primary step for providing peace on the Korean peninsula."

North Korea, backed by China, fought the South and US-led UN forces in the 1950-53 Korean War which ended in a mere truce, not a peace treaty.