SEOUL : South Korean President Moon Jae-in said on Monday ties with Japan are being blocked by historical issues that will take time to resolve and he asked for Japan's understanding and cooperation on the issue.

Moon's comments were an apparent reference to Korean "comfort women", a Japanese euphemism for women forced to work in the Japanese military's wartime brothels. The issue of the women has plagued bilateral ties between the neighbours for decades.

Moon told the visiting secretary general of Japan's ruling Liberal Democratic Party that the people of South Korea did not accept a deal reached by his conservative predecessor and Japanese Prime Minister Shinzo Abe in 2015 to resolve the issue. But Moon, in his talks with Toshihiro Nikai, apparently did not directly address whether he would seek to renegotiate the agreement, in which Japan made an apology to the women, who are now elderly, and promised about one billion yen ($9.07 million) for a fund to help them.

The two governments agreed under the deal that the issue would be "irreversibly resolved" if both sides fulfilled their obligations.

"Both South Korea and Japan should look at this issue directly and understanding is needed that it will take more time (to resolve it)," Moon told Nikai, the South's presidential office said.

Moon, who suggested during his successful campaign for a May 9 election that he could try to renegotiate the deal, also said the two countries should not "cling to past history" only to block other developments in their ties.

Moon stressed the importance of cooperation with Japan in efforts to denuclearise North Korea, which has been ramping up weapons tests since last year in defiance of global sanctions.

Nikai said he agreed with Moon and hoped the two countries could move forward together, the South's presidential office said.

The meeting came hours after the South's foreign ministry warned Japan to exercise caution when making remarks about bilateral ties, including the issue of the women, following reported comments by Nikai in a meeting with South Korean lawmakers.

During the meeting, Nikai said "those plotting schemes" to block bilateral ties from moving forward should be "eradicated", several Japanese and South Korean media reported.

"I don't know if in South Korea there are even a handful, but they must be eradicated," Nikai was quoted as saying.

He also expressed hope that the two countries would "get along well" and not to have relations tangled with "trivial matters".

Nikai's office at the Liberal Democratic Party could not confirm his reported comments, which appeared not to include any direct reference to the women who were forced to work in Japanese brothels. But he came under fire from South Korean civic groups for what appeared to be criticism of people who support scrapping the 2015 agreement.

"Comments related to relations between South Korea and Japan, including those regarding the comfort women issue, should be made with care," a South Korean foreign ministry official said, in response to Nikai's reported comments.

The official declined to be identified citing sensitivity of the issue.

Nikai, the number two executive in his party behind the prime minister, is known for his generally warm ties with South Korea and China.

Some of the women have rejected the agreement and asked for a personal apology from Abe himself, among other demands.

Abe and other Japanese officials have repeatedly said it was vital for the 2015 agreement to be implemented and for the two sides to move ahead based on a future-oriented relationship.