U.S. President Donald Trump said Tuesday he raised the issue of North Korea's past abductions of Japanese nationals during his unprecedented meeting with North Korean leader Kim Jong Un in Singapore.

"I brought it up absolutely and they are going to be working on that," Trump said at a news conference following his historic summit with Kim in the city-state, when asked whether the abduction issue came up during their meeting.

He added, "We didn't put it down in the document but it will be worked out," referring to the joint statement signed by the leaders and released after their meeting.

The statement did not touch on human rights in North Korea, including the abductions of Japanese citizens.

Tokyo had asked Trump to take up during the summit with Kim the long-standing issue involving Japanese citizens abducted by North Korea in the 1970s and 1980s. Pyongyang claims the issue has been settled.

Trump reassured Japanese Prime Minister Shinzo Abe during a phone call Monday that the issue would be taken up. Abe said afterward that Trump had promised the issue would be raised "100 percent" in the meeting with Kim.

Abe and Trump also met in Washington last Thursday. At their joint press conference in the U.S. capital, Abe expressed eagerness to meet with Kim, saying, "I would like to directly face North Korea and talk with (Kim) so as to achieve an early resolution of the abduction issue."

Abe has made resolving the abductions a top priority of his administration.

(Trump met relatives of Japanese national abducted by N. Korea during his visit to Tokyo in November 2017)

Japan officially lists 17 citizens as abduction victims and suspects North Korea was involved in many more disappearances. Five of the 17 were repatriated in 2002, with Pyongyang maintaining that eight had died and four others never entered the country.

Japan and North Korea agreed in 2014 that Pyongyang would reinvestigate the fate of all of the abductees. But the North later disbanded the investigation panel and effectively abandoned the bilateral accord.

The decades-old issue remains a stumbling block for Japan and North Korea to normalize diplomatic ties.

Amid a flurry of diplomacy related to denuclearizing North Korea, there had been concern that bringing up what some view as a purely Japanese issue could stymie global efforts to compel the North to give up its nuclear weapons.

Japan has maintained there can be no normalization of bilateral ties or extension of economic support unless the abduction, nuclear and missile issues are resolved in a comprehensive manner.