A Luxembourg court has blocked a long-running US request to transfer $1.6 billion of dollars in Iranian assets to victims of the September 11 attacks, an official statement said on Monday.

The decision confirmed a claim by Iran's President Hassan Rouhani who on Sunday said the country had won a legal victory over the assets that had long been frozen on a US request in Luxembourg.

The official statement in Luxembourg said that an appeals court on April 1 found the US seizure demand "inadmissible" since the type of account in question is "unseizable" according to national law.

The money is held in the Clearstream clearing house, a financial company owned by Deutsche Boerse based in Luxembourg.

However, the statement added that the ruling was not final and could be appealed at Luxembourg's highest court.

In a separate decision, the statement said a Luxembourg district judge on April 3 blocked the transfer of funds and said Clearstream would be subject to a daily fine of 1 million euros ($1.09 million) if it moved the money.

An attempt on April 7 by Clearstream to have the transfer ban lifted was refused by the president of the Luxembourg court on procedural grounds.

Tehran and Washington have long been arch enemies and tensions have risen sharply since President Donald Trump in 2018 withdrew from a nuclear accord and reimposed stinging sanctions.

In this separate dispute, a New York court in 2012 ordered Iran to pay $7 billion in damages over the September 11 attacks, arguing that it had aided Al-Qaeda by allowing its militants to travel through its territory.

Iran has rejected the accusation and refused to pay the money leading US authorities to demand asset seizures wherever they can.

Rouhani said in a televised cabinet meeting Sunday that "our central bank, our foreign ministry [have] recently won a very good victory in a legal battle".

"$1.6 billion of our money was in Luxembourg and the Americans had put their hands on it," he said.

After trying for months, "we succeeded some days ago and freed this money from the Americans' grasp," he declared.