By Peter Apps

LONDON, Nov 3 (Reuters) - Britain should look at trying ex-Sri Lankan Tamil Tiger rebel Karuna Amman, detained on immigration charges, for war crimes, Human Rights Watch said on Saturday.

The former eastern Liberation Tigers of Tamil Eelam (LTTE) commander is accused by rights experts of child soldier recruitment, extortion and torture both before and after he split from the mainstream rebels in 2004.

Britain's Home Office said late on Friday Karuna was detained after a joint operation between the police and immigration authorities, giving no further details. Human Rights Watch said the authorities should look at doing more.

"Give the magnitude of Karuna"s crimes over the years, including attacks on civilians and use of child soldiers, Human Rights Watch strongly urges the UK government to explore the possibility of prosecuting him for war crimes and other international offences before returning him to Sri Lanka," Human Rights Watch senior legal adviser Jim Ross told Reuters.

The Home Office was not immediately available to comment. Britain is a signatory to the International Criminal Court, which has charged a Congolese rebel with child recruitment, but Sri Lanka is not and the court is not investigating the conflict in the island. Ross said he was unsure if Karuna could be charged under UK law.

Since the split, Karuna has been fighting his former colleagues in the east as the island's 2002 ceasefire collapsed into outright war. Analysts and diplomats say the government has been using and supporting him, a charge both he and the government denied.

His attacks on the mainstream rebels were seen as a factor in rising violence last year. Experts estimate some 5,000 people have died since last year bringing the total death toll from two decades of civil war to some 70,000.

Some fear further escalation after the mainstream Tiger political leader S.P. Thamilselvan was killed in a government air strike on Friday. The Tigers have vowed retaliation.

Analysts say Karuna was used by the government as they cleared mainstream Tiger forces from the east this year. Rights groups say troops and police did nothing to stop his fighters carrying out killings and extortion and taking children to fight.

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