President Hassan Rouhani insisted Wednesday that Iran would not abandon its enrichment of uranium, after US senators called for it to be denied any such right under a long-term nuclear deal.

"The world has admitted that Iran is, and will be, among the countries which have nuclear technology, including enrichment, and there is no doubt about this for anyone," state media quoted Rouhani as telling a cabinet meeting.

His comment came after an overwhelming majority of US senators signed a bipartisan letter to President Barack Obama on Tuesday urging him to reject Iran's claim to the right to enrich uranium for civilian purposes in talks under way with the major powers.

"We believe that Iran has no inherent right to enrichment under the nuclear Non-Proliferation Treaty," the letter signed by 83 of the 100 members of the US Senate said.

Rouhani said Iran was ready to be more transparent about its nuclear programme to allay Western concerns about its ambitions.

"We do not want to make anybody worried... today we are negotiating for a final agreement which is reachable within six months," he said.

The latest round of talks between Iran and the six powers wrapped up on Wednesday, with EU foreign policy chief Catherine Ashton describing them as "substantive and useful".

The next round is scheduled for April 7 in the quest for a long-term deal by a July 20 target date set under an interim agreement reached last November.

The six powers -- Britain, China, France, Russia and the United States plus Germany -- want Iran to reduce permanently, or at least for a long time, the scope of its nuclear activities in order to make it extremely difficult for it ever to develop nuclear weapons.

This would likely include Iran slashing the number of centrifuges enriching uranium -- which can be used for peaceful purposes but also in a bomb, if highly purified -- and allowing tougher UN inspections.