* S.Korea says 'comfort women' a crime against humanity

* Moon says Japan can not declare the issue settled

* Japan protests, says comments 'regrettable'

* S.Korea, Japan key allies in containing N.Korea threat (Updates quotes, adds context)

By Josh Smith

SEOUL, March 1 (Reuters) - South Korean President Moon Jae-in described Japan's wartime use of "comfort women" as a "crime against humanity" on Thursday in some of his strongest comments yet, sparking an immediate protest from his key ally in containing North Korea.

Moon said during a speech marking a national holiday commemorating Korean resistance to Japanese occupation - his first since taking office last year - that Japan was in no position to declare the emotionally charged issue settled.

"To resolve the comfort women issue, the Japanese Government, the perpetrator, should not say the matter is closed," Moon said.

"The issue of a crime against humanity committed in time of war cannot be closed with just a word. A genuine resolution of unfortunate history is to remember it and learn a lesson from it."

His comments drew an immediate rebuke from Tokyo.

Japanese Chief Cabinet Secretary Yoshihide Suga described Moon's comments as "extremely regrettable".

Suga, speaking at a regular briefing, also urged cooperation between South Korea and Japan to tackle the threat posed by North Korea.

The two Koreas have pursued a thaw in relations that began ahead of last month's Winter Olympics in South Korea, but Seoul remains a key part of the international push to increase pressure on Pyongyang to abandon its nuclear weapons and ballistic missile programmes.

Japan and South Korea share a bitter history that includes Japan’s 1910-45 colonisation of the peninsula and the use of "comfort women", Japan's euphemism for women - many of them Korean - forced to work in its wartime brothels.

Japan apologised to the women and provided a 1 billion yen ($9.4 million) fund to help them under a 2015 deal with Moon's conservative predecessor, but South Korea has recently sought to revisit the issue.

"These details were agreed by South Korea and Japan and we find it unacceptable and extremely regrettable," Suga said.

HISTORIC TENSIONS

Moon, speaking at the site of a former jail where Korean independence fighters were imprisoned by Japanese forces, said South Korea was not looking for "special treatment" from Tokyo.

However, he hoped Japan pursued "sincere self-reflection" and "squarely face the truth of history and justice with the universal conscience of humanity".

Japan also formally complained on Monday after South Korea's foreign minister raised the issue at the top U.N. rights body, warning that it should not be allowed to harm bilateral relations at a critical time in East Asia.