"As founding shareholders of the Instrument in Support of Trade Exchanges (INSTEX), France, Germany and the United Kingdom warmly welcome the decision taken by the governments of Belgium, Denmark, Finland, the Netherlands, Norway and Sweden, to join INSTEX as shareholders," the three said in a joint statement.

The Paris-based INSTEX functions as a clearing house allowing Iran to continue to sell oil and import other products or services in exchange.

The system has not yet enabled any transactions.

Washington in 2018 unilaterally withdrew from the international agreement governing Iran's nuclear programme and reinstated heavy sanctions against Tehran.

The accession of the six new members "further strengthens INSTEX and demonstrates European efforts to facilitate legitimate trade between Europe and Iran", France, Germany and Britain said.

It represents "a clear expression of our continuing commitment to the Joint Comprehensive Plan of Action" -- the 2015 Iranian nuclear deal -- the trio added.

They insisted Iran must return to full compliance with its commitments under the deal "without delay".

"We remain fully committed to pursuing our efforts towards a diplomatic resolution within the framework of the JCPoA."

The 2015 deal set out the terms under which Iran would restrict its nuclear programme to civilian use in exchange for the lifting of Western sanctions.

Since the US pullout, Iran has taken four steps back from the accord.

The latest was on November 4 when its engineers began feeding uranium hexafluoride gas into mothballed enrichment centrifuges at the underground Fordow plant south of Tehran.