Japan's top government spokesperson says his government will closely watch the impacts of the US withdrawal from the World Health Organization.

Newly sworn-in US President Donald Trump signed an executive order on Monday to begin the process of withdrawing from the UN body.

Chief Cabinet Secretary Hayashi Yoshimasa told reporters on Tuesday that Japan's position toward the WHO remains unchanged despite the US move.

He went on to say that Japan expects the body to continue playing its role of addressing various problems based on scientific knowledge and findings.

Hayashi also called the US an important contributor to international health and said Japan will keep working with the country.

Trump also labeled North Korean leader Kim Jong Un a "nuclear power" as he spoke to reporters at the White House on Monday. He added that he and Kim "got along very well"

Regarding the comments, Hayashi said North Korea's nuclear and missile development programs are totally unacceptable as they threaten the peace and security of Japan and the international community.

He went on to say Japan will continue working closely with the US and other countries to deal with the North on issues including its abductions and nuclear and missile development programs.