* U.S. says not forewarned of 'disappointing' S.Korean decision
* S.Korea asked U.S. ambassador to tone down comments - source
* Japan, S.Korea in deepening political and economic feud
* S.Korean veterans group cancels U.S. ambassador's speech
By Josh Smith and Hyonhee Shin
SEOUL, Aug 29 (Reuters) - South Korea's decision to scrap a military intelligence-sharing pact with Japan has led to an increasingly public split with the United States, just when the allies face rising tensions with North Korea and new competition from China and Russia.
South Korean First Vice Foreign Minister Cho Sei-young met U.S. Ambassador Harry Harris on Wednesday and asked that the United States' tone down its public criticism of South Korea's decision not to renew the General Security of Military Information Agreement (GSOMIA) with Japan, according to a diplomatic source.
Cho called the U.S. criticism unhelpful and asked that Washington "refrain from giving such public messages", the source said, speaking on condition of anonymity.
A South Korean veterans group later cancelled a speech by Harris scheduled for Thursday, citing the "rapidly changing security circumstances".
"As a security organisation, we wanted to hear about North Korea's denuclearisation and the issues of the alliance from him, but we found the timing did not quite fit because all the attention is focused on the GSOMIA issue," a group representative told Reuters.
A spokesman for the U.S. Embassy in Seoul said he could not discuss the contents of private diplomatic conversations and referred questions back to the veterans group.
South Korea announced last week it would not renew GSOMIA, citing a "grave change" in the environment for bilateral security cooperation caused by Japan's decision to remove South Korea's fast-track trade export status.
Japanese officials have said South Korea's decision showed a failure to appreciate the growing threat from North Korea.
Defence Minister Takeshi Iwaya said on Tuesday bilateral and trilateral cooperation between Japan, South Korea, and the United States was crucial and urged Seoul to reconsider scrapping the ban before it officially expires in November.
'NOT FOREWARNED'
U.S. officials had largely stayed on the sidelines of the spiralling dispute but expressed concern and disappointment with Seoul's decision not to renew the GSOMIA pact. It also caught them off guard.
"In terms of the actual decision to not renew, we were not forewarned," Randall Schriver, U.S. assistant secretary of Defense for Indo-Pacific Security Affairs, said on Wednesday.