Hiroshige Seko, Abe's assistant in charge of public relations, made the remark after the premier sparked controversy by saying there was no evidence of coercion in the case of "comfort women."

"There are various definitions of coercion, with some being strict and others being more broad," Seko said in a popular talk show on the TV Asahi network.

But he added that Abe's position "has not changed."

In 1993, the then chief cabinet secretary, Yohei Kono, the top government spokesman, issued a statement voicing "sincere apologies and remorse" and acknowledging that Japan's imperial army was involved "directly or indirectly" in sexual slavery.

Historians say at least 200,000 young women, mostly from Korea but also from China, Indonesia, the Philippines and Taiwan, were forced to serve as sex slaves in Japanese army brothels.

But Abe, who is known for his conservative views on history despite his efforts to improve relations with Asian countries, has voiced doubts that Japan forced women into sexual slavery.

"The fact is that there is no evidence to support the claim of the coercion," Abe said late Thursday.

South Korean Foreign Minister Song Min-Soon told reporters during a visit to Washington on Friday that the comment was "not helpful" to bilateral efforts to build a better relationship.

Abe's comment has also been widely published by foreign media, including major US newspapers, as the US Congress is preparing to discuss the issue.

A bill -- sponsored by Representative Mike Honda, a Democrat who spent part of his childhood in a wartime internment camp for Japanese-Americans -- demands an apology by Japan and outright recognition of its involvement.