The key to improvement in relations between South Korea and Japan is for Tokyo not to backtrack on past apologies for its imperialist past and crimes committed during that period, a former senior U.S. diplomat said.

Stephen Bosworth, who served as ambassador to South Korea, made the remark during an interview with Yonhap News Agency, highlighting the difference between the ways Japan and Germany have dealt with historical issues.

"I think the key here is Japan should not go back on what it has already agreed to in the form of various statements from the Japanese government stretching back 15 or 20 years on the subject of comfort women and other subjects," Bosworth said.

Under Prime Minister Shinzo Abe, Japan has taken a series of nationalist steps, including an attempt to review a 1993 statement of apology for the country's sexual enslavement of women for its troops during World War II. The move was heavily denounced as an effort to undermine the apology.

Such moves have led to Japan's relations with South Korea plunging to one of the worst points.

"That's why some of the statements that have been attributed to Prime Minister Abe and some of the statements by his supporters and advisers have not been helpful because they appear to be an effort to redefine some of the statements that have been made previously and I don't think that's in Japan's interest," Bosworth said.

Abe's historical perceptions will be a key focus of attention in an address he is expected to deliver at a joint session of Congress when he visits Washington next month. Abe would be the first Japanese leader to address a joint meeting of the House and Senate.

"I don't know what he will do and I don't know if he will speak," Bosworth said. "I am interested, though, to see the difference between the way that Japan has dealt with this issue over the decades since World War II and the way that Germany has dealt with this issue."

In the German case, historical issues are "not any longer an obstacle or even a consideration in Germany's relationships with its European neighbors or with the U.S. or anyone else," he said.

"I would think that that would be, has been, should be Japan's objective, to eliminate this as a source of tension in the relationships -- and frankly, I think that the steps that Japan has taken over the last couple of years have been in the wrong direction," he said.

Bosworth, who retired from the Foreign Service as special representative for North Korea policy, also expressed concerns about the lack of dialogue with Pyongyang, stressing there is nothing constraining the North's nuclear development.

"What is important is the fact that North Korea continues to develop nuclear weapons. They are unconstrained. At least in the past, when we've been talking to them they had not been conducting tests and they had frozen the programs that we knew about, at least. Now, they have no constraints at all," Bosworth said.

"I think all the experts agree that in five years they could have many more nuclear weapons then they might have now. To deny that is to simply deny reality," he said.

In January, Bosworth, former U.S. nuclear negotiator Joseph DeTrani and some American scholars held unofficial "Track 2" meetings with North Korea's chief nuclear envoy, Ri Yong-ho, and other diplomats in Singapore.

Bosworth said he had useful conversations where the North Koreans complained about U.S. government policy and the American participants explained to them that the U.S. is not hostile toward Pyongyang, but has a problem with their pursuit of nuclear weapons.

Ri also told the Americans that the nuclear program is very important for the country, but that once formal negotiations reopen, "everything would be on the table," according to Bosworth.

The remark, however, does not represent a commitment to eliminate the nuclear program, he said.

Six-party talks have been stalled since the last session in late 2008. North Korea has called for unconditional resumption of the negotiations, but the U.S. has demanded Pyongyang first take steps demonstrating its denuclearization commitments.

Bosworth said it takes both sides to make some movement to restart the negotiations, but most of the movement should come from the North as it was the one that violated the so-called "Leap Day agreement" of 2012.

Under the agreement, the North promised to halt nuclear and missile tests and allow U.N. nuclear inspectors back into the country in exchange for 240,000 tons of food aid from the U.S. But the deal fell apart two months later when Pyongyang conducted a long-range rocket launch.

"I think they have to in effect indicate where we start from," Bosworth said. "We are not going to start from before the Leap Day agreement, so you really have to start from the Leap Day agreement and move on and I would think that is where we should be trying to go."

Bosworth said he does not have any plans to have such "Track 2" meetings with the North Koreans in the foreseeable future.

"I would very much hope that the U.S. and North Korea could come to a point which they could resume government to government conversations. I don't know if that is going to happen soon. I think it will happen eventually," he said.

"Once that happens then the need for people like myself -- the value for people like myself in meeting with the North Koreans largely disappears, he added. (Yonhap)