The United States and North Korea will likely start preparing a joint statement of their leaders to be issued at their second bilateral summit scheduled for next month, the head of Seoul's state spy agency said Tuesday.

Suh Hoon, director of the National Intelligence Service (NIS), said he expected the two sides to begin discussing major topics for the proposed summit between US President Donald Trump and North Korean leader Kim Jong-un.

"I believe they will begin coordinating topics for the summit in order to draft a joint declaration in the North Korea-US working level negotiations, along with their working-level preparations for the second North Korea-US summit such as safety and protocols," the NIS chief was quoted as telling the parliamentary intelligence committee.

Trump and Kim issued a joint declaration calling for complete denuclearization of North Korea when they first met in Singapore in June.