In a statement, the Treasury Department said it has placed sanctions on Fazl-ur Rehman Khalil, a founder of Harakat ul-Mujahadeen, or HUM, which was listed by the US State Department in 1997 as a foreign terror organisation.
The terror group operates throughout India, Pakistan and Afghanistan. Last year, the group renamed itself Ansar al-Umma. The US State Department amended its listing of HUM this year to include the new name.
He kept a close relationship with al-Qaeda, including with its chief Osama bin Laden until his death, it added.
The Treasury also placed sanctions on Muhammad Naeem Sheikh and Umair Naeem Sheikh for acting on behalf of, and for providing financial support to LeT.
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Naeem has been a member of LeT since its early days, and he has been responsible for soliciting wealthy businessmen to fund the group, the statement said.
Umair has been one of the LeT's "most significant financial supporters", Treasury said, saying he has served as a financial courier to the head of its Afghanistan operations.
The Treasury action freezes their assets in any US jurisdiction and bans US citizens from doing business with them.