The group, 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.

LTTE has "quite a resume" as a terrorist group that 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" has inspired terrorist networks worldwide, including al Qaeda in Iraq, it added.

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

The FBI did not mention the names but 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 LTTE amidst fears that the conflict could escalate in coming days.

In its campaign to seize control of the country from the Sinhalese ethnic majority and create an independent Tamil state, 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, FBI added.

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," FBI asserted.

In April, for example, 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.