* Moon says 2015 agreement "cannot solve" issue
* S.Korea, Japan had agreed to a resolution
* Kyodo says now difficult for PM Abe to visit South Korea (Adds report PM Abe's trip to Winter Olympics now difficult)
By Joyce Lee and Hyonhee Shin
SEOUL, Dec 28 (Reuters) - South Korean President Moon Jae-in said on Thursday that a 2015 agreement with Japan over South Korean "comfort women" forced to work in wartime brothels was seriously flawed after Japan said any attempt to revise it could damage relations.
A South Korean panel set up to investigate the deal concluded on Wednesday that it failed to meet the needs of the thousands of girls and women forced to work in Japan's military brothels, many of them Korean, euphemistically termed "comfort women" by Japan.
The announcement threw ties into doubt as both countries, important U.S. allies, seek to rein in North Korea's nuclear and missile programmes.
"The agreement cannot solve the comfort women issue," Moon said, calling the deal a "political agreement that excludes victims and the public" and violates general principles in international society, according to a statement issued by his office.
A Japanese foreign ministry spokeswoman said Japan had conveyed its position to South Korea through diplomatic channels following Moon's remarks, reiterating Foreign Minister Taro Kono's comment on Wednesday that any attempt to change the deal would be "unacceptable" and make relations "unmanageable".
Japan's Kyodo news agency quoted an unnamed Japanese government source as saying it had now become difficult for Prime Minister Shinzo Abe to visit South Korea in time for the Winter Olympics in Pyeongchang in February, in a potential sign of chilling ties.
Four years ago, Abe took a trip to Russia's Black Sea resort of Sochi to attend the opening ceremony of the Winter Olympic Games there and held talks with President Vladimir Putin.
Asked if Moon meant to declare the deal null and void, Park Soo-hyun, a spokesman for the presidential Blue House, said it was "inappropriate" for him to use that term at this point, adding the government would present its "final position".
Under the 2015 deal, Japan apologised to victims and provided 1 billion yen ($8.8 million) to a fund to help them.
The two governments had agreed the issue would be "irreversibly resolved" if both fulfilled their obligations.
Moon pledged to normalise relations and work toward "future-oriented cooperation" with Japan.
Japan's Nikkei business daily on Thursday quoted Abe as telling people close to him that the agreement "will not be changed by even one millimetre".
"Regardless of the Japanese government's stance, we take the investigation results seriously and humbly," South Korean foreign ministry spokesman Roh Kyu-deok told a news briefing, adding Seoul would formulate follow-up measures as soon as possible that could help the victims "regain honour and heal the wounds in their hearts".
Japan and South Korea, which share a bitter history including Japanese colonisation, are key to international efforts to rein in North Korea's nuclear and missile programmes that it pursues in defiance of U.N. Security Council resolutions.
The comfort women issue has regularly been a source of rancour between Japan and neighbours China and North and South Korea. Japan colonised the Korean peninsula between 1910 and 1945 and occupied parts of China before and during the war.
The legacy of colonial rule, especially the comfort women issue, remains highly sensitive in South Korea, while in Japan, some ultra-conservatives deny that the women were forced to work in brothels at all.
In 2014, the U.N. Human Rights Committee asked Japan to clarify the "comfort women" euphemism, with an independent expert on the panel calling for it to be replaced by "enforced sex slaves".
The liberal Moon came to power in May winning a snap election called after the removal of his disgraced predecessor, Park Geun-hye, whose conservative government was criticised for failing to fully consult victims ahead of the 2015 settlement.
Moon pledged to renegotiate the agreement while on the campaign trail.
A poll in December last year showed 59 percent of South Koreans thought the deal should be nullified while 25.5 percent supported it.
Park was forced from power this year over a corruption scandal and is now on trial. (Reporting by Joyce Lee and Hyonhee Shin; Additional reporting by Hyunjoo Jin in Seoul and Linda Sieg, Kiyoshi Takenaka in Tokyo; Editing by Nick Macfie)