Japan and North Korea held secret talks in mid-May in Ulan Bator over North Korea's abductions of Japanese nationals decades ago, a diplomatic source in Mongolia said Sunday.

Officials of the two countries met secretly before what Japan viewed as the one-year deadline on July 4 for completing the North Korean probe. North Korea told Japan on July 2 it needed more time to complete the investigation, according to the Japanese government.

The investigation is the third that Pyongyang has promised to conduct, following probes in 2002 and 2004 that Japan found unconvincing.

Prime Minister Shinzo Abe sent an aide to Mongolia in mid-May to seek its help in getting North Korea to clarify what became of Japanese nationals it abducted in the 1970s and 1980s, a source familiar with Japan-North Korea relations said. Chances are high that the aide, one of Abe's executive secretaries, joined the secret talks.

In July last year, North Korea launched a comprehensive investigation into all Japanese residing in the country in return for Tokyo lifting some sanctions. The probe included a reinvestigation into the 12 Japanese citizens officially listed by Tokyo as abduction victims who are still missing.

On Sunday, Shotaro Yachi, Abe's key foreign policy adviser who heads the secretariat of Japan's National Security Council, held talks in the Mongolian capital with Tsagaandari Enkhtuvshin, secretary of Mongolia's National Security Council.

The Mongolian official told reporters he promised Yachi that Ulan Bator will continue to serve as a mediator for the settlement of the abduction issue. Mongolia has diplomatic ties with North Korea, while Japan does not.