A Christian humanitarian group that has for decades delivered aid to Cuba has been threatened with having its US tax exempt status stripped.  in a move that has been condemned by .

The Cuban Council of Churches has condemned the move against Pastors for Peace, which has defied Washington's sanctions by travelling to Cuba to deliver aid from the US since 1992.

The organisation collects donations from across America and sends school buses, computers, medicine and other goods to Cuba.

It describes itself as an "interfaith organisation to assist oppressed peoples in their fight for justice and self-determination".

Its Cuba deliveries are a "nonviolent direct challenge to the brutal US economic blockade of Cuba", it says.

"We act not just in defiance of our government, but in obedience to our conscience," said Rev Lucius Walker, the founder Pastors for Peace.

Last month, the Internal Revenue Service (IRS) notified the group's parent organisation, the Inter-religious Foundation for Community Organization, that its status was in jeopardy because it had never requested permission from the US Treasury Department to send aid. The IRS said it had been investigating the group since 2009.

"I do not understand how at this moment, when the Obama Administration's policy is to seek understanding, that on the other hand they are taking these types of measures against institutions that have created an understanding between our peoples," Joel Ortega Dopico, President of the Cuban Council of Churches, said at a press conference in Havana.

Cuba and the United States began normalising relations in December 2014 after 18 months of secret talks and have since restored full diplomatic ties. The countries had been hostile for more than five decades, since Fidel Castro ousted US-backed dictator Fulgencio Batista in a 1959 revolution that steered the island on a communist course and made it a close ally of the Soviet Union during the Cold War.

There have been numerous skirmishes between Pastors for Peace and US authorities at the border, temporary detentions and confiscations of aid over the years, but never prosecutions or other legal actions taken against the group or its members.

"It is not up to the IRS to decide what we can bring to Cuba and in what way," Manolo de los Santos, in charge of Cuba for Pastors for Peace, said in Havana.

Additional reporting by Reuters.