US President Mr George W Bush today designated the "Real IRA" a 'terrorist organisation', a legal description resulting in financial and other sanctions.
A notice in the Federal Register said Secretary of State Mr Colin Powell made the designation, which takes effect today.
Assets the "Real IRA" holds in the US are now frozen under law, it is illegal to support the organisation and members are not eligible for US visas.
The "Real IRA" was responsible for the 1998 Omagh bombing that killed 29 people.
Speaking earlier today, Sinn Féin president Mr Gerry Adams said the ban would not have any impact.
He said: "I have to say that I don't think it will have any effect at all. It's a symbolic measure. If anything, it may have the effect of giving these people an exaggerated sense of their own importance."
He said: "What is required back here in Ireland is real alternatives to be developed and worked on. We are saying to people 'don't support these groups'. They don't have a strategy. They don't have any possibility of succeeding. We're the real republicans. We have a real strategy. You can see progress and you can be a part of it."
Northern Ireland's first minister Mr David Trimble welcomed the decision taken by the US to officially ban the Real IRA, saying it had a symbolic and practical impact.
"This development is important at a symbolic level. It sends a strong and clear signal that time has run out for Irish Republican terrorist organisations.
"It will deny the Real IRA an important source of fundraising and will refuse their apologists and supporters access to the USA," said the leader of the Ulster Unionists, Northern Ireland's main Protestant party.
Mr Victor Barker, whose son James was among those killed in the Omagh explosion in August l998, had earlier said: "They ["The Real IRA"] are being banned because they are a terrorist organisation. Remember, they kill little children," he said. PA