(Updates with details)

By Ranga Sirilal

COLOMBO, May 20 (Reuters) - A senior Tamil Tiger military commander died from a heart attack in Sri Lanka's rebel-held north on Tuesday, pro-rebel websites and radio said.

"Brigadier Balraj, a senior and special commander of the Liberation Tigers of Tamil Eelam (LTTE) passed away Tuesday at 2 p.m. of a sudden heart attack in Vanni," said pro-rebel website www.tamilnet.com, quoting the LTTE headquarters.

The rebels have declared three days of mourning.

Balraj was the first commander of the rebels' Charles Anthony special brigade, an elite commando unit established in 1991. He served as a key commander for the LTTE's major operations.

He is the most senior figure to die since Tamil Tiger political-wing leader S.P. Thamilselvan was killed in air force bombing raids in November last year.

Rebel chief negotiator and ideologue Anton Balasingham died of cancer in London in December 2006.

The Tamilnet website and military sources said Balraj was an expert in psychological operations. He had heart surgery in Singapore in 2003 during the last ceasefire with the government and had been hospitalised for three months earlier this year.

Fighting between government forces and the LTTE has intensified since the government formally pulled out of a six-year-old ceasefire pact in January, though a renewed civil war has been raging since 2006.

The rebels are fighting for an independent state for minority Tamils in north and east and more than 70,000 people have died in the 25-year-old conflict.

Charles Anthony died in fighting during the early 1980s and is regarded as a hero to the rebels, so much so that LTTE leader Velupillai Prabhakaran named his son after him. (Editing by David Fogarty)

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