The 2007 cable, written by the then US ambassador to Sri Lanka, Robert Blake, cited evidence provided by "trusted embassy contacts" that top officials allowed the groups to operate with impunity, and even encouraged them.
"It appears that this involvement goes beyond merely turning a blind eye to these organizations' less savory activities," said the cable released by whistleblowing website WikiLeaks.
"At worst, these accounts suggest that top leaders of its security establishment may be providing direction to these paramilitaries."
Blake is currently the US Assistant Secretary for South and Central Asian Affairs.
While some military commanders wanted to crack down on the paramilitary groups, the cable cited sources as saying Defence Secretary Gotabhaya Rajapakse -- the brother of President Mahinda Rajapakse -- had ordered them not to interfere.
The order was made "on the grounds that they are doing work that the military cannot do because of international scrutiny," it s