South Korea's presidential office Cheong Wa Dae expressed optimism on Saturday about US President Donald Trump's recent announcement that he would speak with North Korean leader Kim Jong-un on the phone, calling it "very important progress."

On Friday, Trump said he gave Kim a direct phone number to reach him and that he plans to personally call Kim on Sunday (US time).

"Most important of all, this means the US and North Korean leaders are in a relationship where they can talk on the phone whenever needed," a high-ranking presidential official told Yonhap News Agency on the phone, asking not to be named.

"It is a symbolic event for progress in relations between the two countries that they can discuss pending issues whenever they want," he said.

The official predicted that if the two leaders make a phone call as planned, they would mainly discuss how to implement the joint statement adopted during their June 12 summit in Singapore.

Trump and Kim declared in the joint statement following the summit their commitment toward the complete denuclearization of the Korean Peninsula while guaranteeing the security of the North Korean regime. (Yonhap)