Describing the Liberation Tigers of Tamil Eelam as the 'most dangerous and deadly extremist' outfit in the world, the United States' Federal Bureau of Investigation has asked donors to be careful about charities established as front organisations by the rebel group to raise funds to finance its terrorist activities.

The group, the FBI said, has placed operatives right in the United States to discreetly raise money to purchase weapons and explosives and to fund its 'terrorist campaign' in Sri Lanka [Images].

The LTTE has 'quite a resume' as a terrorist group, which is credited with perfecting the use of suicide bombers, inventing suicide belts and pioneering the use of women in suicide attacks, the top investigating agency said.

The rebel group's 'ruthless tactics' have inspired terrorist networks worldwide, including Al Qaeda [Images] in Iraq, it added.

Stating that the LTTE has killed some 4,000 people in its campaign of 'violence and bloodshed' in the last two years, the FBI said its operatives have assassinated two world leaders, 'the only terrorist organisation to do so'.

The FBI did not mention the names, but the LTTE is known to be behind the assassinations of Rajiv Gandhi in 1991 and Sri Lankan president Ranasinghe Premadasa in 1993.

The FBI warning comes within days of the Sri Lankan government abrogating a ceasefire agreement with the LTTE amidst fears that the conflict could escalate in the coming days.

In its campaign to seize control of the country from the Sinhalese ethnic majority and create an independent Tamil state, the FBI said, "The LTTE has launched suicide attacks, assassinated politicians and committed all kinds of crimes to finance its operations.

The resulting civil war has taken the lives of nearly 70,000 Sri Lankans on both sides of the conflict since 1983 alone".

The US government had designated the Tigers as a foreign terrorist organisation. "We're determined to stop them, using the full range of our investigative and intelligence capabilities," the FBI asserted.

"In April 2007, a joint terrorism task force in New York had arrested the alleged US director of the Tigers," it said. The man was alleged to have held several fund-raising events at a church and various public schools in Queens and in northern New Jersey in 2004. He is also accused of arranging high-level meetings between the group's leaders and US supporters.

"We've also arrested another 11 Tamil Tiger-related suspects in the New York City region. And in Baltimore, following a multi-agency investigation, a pair of Indonesian men pleaded guilty and were sentenced recently for working with others to export surface to air missiles, state-of-the-art firearms, machine guns and night vision goggles to the Tigers in Sri Lanka," the FBI said.