Talks between Iran and six world powers on implementing November's deal to freeze parts of Tehran's nuclear programme have stumbled over the subject of advanced centrifuge research, diplomats have said.

A Western diplomat claimed the issue "was among the main factors in stopping the previous technical discussions on December 19-21."

US State Department spokeswoman Jen Psaki said Undersecretary of State for Political Affairs Wendy Sherman is discussing the issue with EU counterpart Helga Schmid and Iran Deputy Foreign Minister Abbas Araqchi in Geneva.

In December, Iran said it wanted to install additional centrifuges which were modified versions of current machines.

But diplomats now say Iran wants to press ahead with the development of even more advanced models.

Iran is already testing several new and more efficient centrifuge models at its Natanz research facility, according to the UN nuclear watchdog.

But Western negotiators say they are uncomfortable with the development.

Nevertheless, Iran says centrifuge research is crucial. "We have to make sure our right to research and development is respected," a government official insisted.

A Western negotiator claimed that despite disagreements, the talks have "actually made pretty good progress."

"The gaps which have been making it difficult to reach agreement clearly reflect attempts of both sides to improve their status at the last stage before signing the final agreement.

"Either way, the two sides are interested in reaching agreement as soon as possible.

"We are still aiming to get the interim agreement started on January 20," he said.