* Congo government calls Colonel Kahasha a 'big fish'
* M23 rebel group denies Kahasha fought for them
* Other rebels linked to recent killings also give up
By Jonny Hogg
KINSHASA, Nov 14 (Reuters) - A leading army mutineer allied to the M23 rebellion in eastern Democratic Republic of Congo has surrendered, the government said on Wednesday, claiming it as a major blow to the insurgents.
Eastern Congo has been swept by violence since the beginning of the year after hundreds of soldiers defected and launched M23, which says it wants to overthrow President Joseph Kabila.
More than 760,000 people have fled their homes since.
Colonel Albert Kahasha, accompanied by 35 fighters from other rebel groups linked to the killings of hundreds this year, surrendered in the South Kivu capital of Bukavu on Monday, government spokesman Lambert Mende said, describing Kahasha as a "big fish".
"He didn't like the direction taken by M23, who he was allied with ... He had been sent to open up another front of fighting in South Kivu," Mende told Reuters, adding that his decision was a blow for the rebels.
M23 spokesman Colonel Vianney Kazarama confirmed links with Kahasha but denied they were fighting together.
"We gave him freedom of circulation in our zone... ideologically he was with us but not in any military sense. If he wants to re-integrate back into the FARDC (national army), that's his problem," Kazarama told Reuters.
Kahasha surrendered along in with commanders from the Raia Mutomboki and the Nyatura armed groups. The United Nations has said the groups were behind a spate of ethnically motivated tit-for-tat massacres between April and September which left at least 264 people dead.
The U.N.'s joint human rights office (UNJHRO) published a report documenting brutal attacks on isolated villages in Masisi territory, North Kivu, with many victims killed by machete blows or burnt alive in their houses.
Nyatura was working alongside the FDLR, an ethnically Hutu Rwandan rebel group operating in Congo, which has been behind some of the region's worst atrocities in recent years.
Raia Mutomboki - meaning "outraged citizens" in the local Swahili language - is an self-defence group which has spread from South Kivu, feeding off widespread xenophobic and anti-Rwandan sentiment.
"The risk of intensification of ethnic violence gives rise to serious concerns for the peace and security of civilians in the region," the report said. (Writing by Richard Valdmanis; Editing by Michael Roddy)