The US State Department has officially designated the Irish dissident republican group the Real IRA, as a foreign terrorist organisation.
This means the group, which has been blamed for bomb attacks in Northern Ireland - including the 1998 Omagh bombing - and mainland Britain, will have any assets it has in America frozen.
A notice in the Federal Register said Secretary of State Colin Powell made the designation, which took effect on Wednesday.
The sanctions also mean:
* Suspected Real IRA activists denied US visas
* Ban on Americans giving them money or support
The US State Department said it was also designating the 32 County Sovereignty Movement - said to be the Real IRA's political front - and the Irish Republican Prisoners Welfare Association.
It said in a statement: "This action makes it illegal for persons in the United States, or subject to US jurisdiction, to provide material support to the Real IRA or any of its named aliases, requires US financial institutions to block assets of the designated groups and enables us to deny visas to representatives of the group."