LONDON, Nov 2 (Reuters) - British police and immigration officials have arrested a breakaway leader from Sri Lanka's Tamil Tiger rebels, the government said on Friday.

Karuna Amman was arrested following a joint operation between Britain's new Border and Immigration Agency and London police, a Home Office (interior ministry) statement said.

"He is now being held in immigration detention (and) it would not be appropriate to comment further," it said.

The Home Office gave no details of where or when Karuna was arrested or what would happen to him.

Karuna, whose real name is Vinayagamoorthy Muralitharan, is a former eastern commander of the Tamil Tigers.

Once a confidant of reclusive rebel leader Velupillai Prabhakaran, Karuna split from the main movement in 2004, taking an estimated 6,000 fighters with him.

A swift Tiger offensive recaptured his territory and he was widely said to have fled into government areas.

Diplomats and analysts say Sri Lanka was using Karuna as part of its fight against the Liberation Tigers of Tamil Eelam (LTTE) or Tamil Tigers, who aim to carve out an independent homeland for the island's Tamil minority.

A 2002 ceasefire collapsed last year, with the island now back in a two decade long conflict that has killed some 70,000 people and to which analysts see no end in sight.

Sri Lanka's government denies backing Karuna's fighters who have attacked the mainstream LTTE and were accused by U.N. children's fund UNICEF of abducting children to fight.

Some Sri Lankan politicians have begun to talk up Karuna and his movement the Tamileela Makkal Viduthalai Pulikal (TMVP) as an alternative political counterweight to the Tigers.

But rights groups accuse them of a string of abuses and extortion and in recent months there have been repeated reports of a split.

The Tamil Tiger's political wing leader and the international face of the group, S.P. Thamilselvan, was killed in a Sri Lankan air force bombing raid early on Friday, in what analysts warned was a body-blow to any hope of ending the conflict soon.

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