VIENNA -- Iran is taking steps to improve its ability to speed up uranium enrichment that could delay implementation of a nuclear deal with six world powers because Tehran's moves are opposed by the United States and its allies.

Iran's nuclear chief, Ali Akbar Salehi, said late Thursday that his country is building a new generation of centrifuges for uranium enrichment but they need further tests before they can be mass produced. His comments appeared aimed at countering criticism from Iranian hard-liners by showing their country's nuclear program is moving ahead and has not been halted by the accord.

But two officials familiar with Iran's nuclear activities said Tehran has gone even further by interpreting a provision of the interim Geneva nuclear deal in a way rejected by many, if not all, of the six powers that sealed the Geneva deal with Iran.

They said Friday that Iranian technical experts told counterparts from the six powers last week that some of the cutting-edge machines have been installed at a research tract of one of Iran's enriching sites. They gave no numbers.

Iran argued that it had a right to do so under the research and development provisions of the Nov. 24 Geneva accord, said the officials, who represent countries that are members of the Vienna-based U.N. nuclear agency monitoring Tehran's atomic activities. They spoke on condition of anonymity because they are not authorized to discuss the closed meetings.

Iran's approach is being hotly disputed by the United States and other representatives of the six powers -- the United States, Russia, China, Britain, France and Germany -- said the officials. They said they have argued that installing any centrifuge that increases overall numbers, particularly a new model, violates Tehran's commitment to freeze the amount and type of enriching machines at Nov. 24 levels.

In commitments under the Geneva accord, Iran agreed to freeze the number of centrifuges enriching uranium for six months and only to produce models now installed or in operation, so it can exchange them piece by piece for any damaged ones. At the same time, the interim deal allows Iran to continue centrifuge research and development.

The disagreement contributed to the decision to adjourn the talks in Geneva last Sunday, the officials said.

On Friday night, calls to officials in Tehran were not immediately returned on Friday, a weekend day in Iran.