Describing the LTTE as the "most dangerous and deadly extremist" outfit in the world, the FBI 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, 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 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" 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.
Dissidents have posited alternative theories ranging from fear of a sea-borne US invasion to establishment of a new dynastic capital in the tradition of Burma's ancient kings.
(Reporting by Aung Hla Tun; Editing by Ed Cropley and Sanjeev Miglani)