North Korean leader Kim Jong Un's powerful sister said Thursday her country would be open to improving ties with Tokyo, even hinting at a possible future invitation to Pyongyang for Japan's leader.

Kim Yo Jong's comments came after Japanese Prime Minister Fumio Kishida said last week that he felt a "strong need" to change the current relationship between Tokyo and Pyongyang.

"I think there would be no reason not to appreciate his recent speech as a positive one, if it was prompted by his real intention to boldly free himself from the past fetters," Kim said in a statement carried by the official Korean Central News Agency.

Kim said the two countries "can open up a new future together" depending on Tokyo's actions, including moving on from the long-running matter of North Korea's kidnapping of Japanese citizens in the 1970s and 1980s.

"There will be no reason for the two countries not to become close and the day of the prime minister's Pyongyang visit might come," she added.

The abductions remain a potent and emotional issue in Japan.

North Korea admitted in 2002 that it had dispatched agents to kidnap 13 Japanese people in the 1970s and 1980s, pressing them into service training its spies in Japanese language and customs.

But suspicions persist in Japan that many more of its citizens were abducted than have been officially recognised.

In a speech at the UN General Assembly last year, Kishida expressed his wish to meet with North Korea's leader "without any conditions", saying that Tokyo was willing to resolve all issues, including the kidnappings.

Japan's former prime minister Junichiro Koizumi paid a landmark visit to Pyongyang while in office in 2002, meeting Kim's father Kim Jong Il and setting out a path to normalise relations in which Japan would offer economic assistance.

The trip led to the return of five Japanese nationals and a follow-up trip by Koizumi, but the diplomacy soon broke down, in part over Tokyo's concern that North Korea was not coming clean about the abduction victims.