COLOMBO, Jan 31 (Reuters) - Sri Lanka's Tamil Tiger have asked the United Nations to recognise their right to sovereignty as a "constructive" approach to ending the two-decade long civil war, the rebel group said.

"There is only one path open to regain the rights of the Tamil people and that is for the international community to recognise the sovereignty of the Tamil nation," B. Nadesan, head of the Tigers' political wing, said in a letter to U.N. Secretary General Ban Ki-Moon.

The letter was released late Wednesday.

While the U.N. has frequently discussed Sri Lanka's civil war at the General Assembly and in the Security Council, Nadesan's letter is believed to be the first formal request for recognition to the world body.

The request is unlikely to be given much official credence -- particularly over Sri Lankan government objections and while the Liberation Tigers of Tamil Eelam (LTTE) remains banned by the United States, European Union and other nations.

In his letter, sent a day after the LTTE blamed Sri Lankan troops for a claymore mine attack which killed 18 civilians, Nadesan also accused the government of deliberately targeting Tamil non-combatants.

"Since the present President of Sri Lanka took office in November 2005, 2,056 Tamil civilians including 132 Tamil children have been massacred by the Sri Lankan state forces," he wrote.

The army and government repeatedly denies targeting civilians.

Analysts say both sides tend to overstate enemy losses and play down their own in a conflict that has killed an estimated 70,000 people.

The majority-Sinhalese government has vowed to wipe out the Tigers militarily and earlier this month scrapped a six-year ceasefire, saying the Tigers had used it to regroup and rearm.

Nordic truce monitors, asked to leave Sri Lanka by the government, have said both sides repeatedly violated the six-year ceasefire. (Reporting by Ranga Sirilal; Editing by David Fox)

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