VIENNA (AP) - 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 hardliners 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 told The Associated Press 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, no one was answering telephone calls for comment at Iran's mission to, the International Atomic Energy Agency, the U.N. nuclear organization. Calls to officials in Tehran were not immediately returned on Friday, a weekend day in Iran. Two technical meetings about Iran's nuclear program since the November agreement have dragged on for several days, but a session planned in Geneva on Monday is scheduled to last only a day. That suggests that both sides are anticipating the need to return to their capitals for more consultations on the issue. The development regarding uranium enrichment reflects the difficulties expected in implementing the Nov. 24 deal as the two sides argue over interpretations of the document outlining both Iran's obligations and moves by the international community to ease economic sanctions in return for Tehran's nuclear concessions. Such differences could delay the deal past its envisaged January start and strengthen both hardliners in Tehran and congressional skeptics in Washington who argue that the accord doesn't work and gives Iran too much for too little in return. In Geneva, Iran also agreed to limit its uranium enrichment to 5 percent and neutralize its stockpile of 20 percent enriched uranium. Depending on its grade, enriched uranium can be used for either reactor fuel or - at levels above 90 percent - for the fissile core of a nuclear warhead. Iran insists it has no interest in nuclear weapons. But the United States and its allies are skeptical. Limiting enrichment with machines that spin at supersonic speed to increase the ratio of fissile isotopes is one of the core aims of the six-month interim deal meant to prepare ground for a permanent accord. Asked for comment on the centrifuge issue, U.S. State Department deputy spokeswoman Marie Harf told the AP that the Geneva plan "makes very clear what Iran is and isn't allowed to do," adding Washington is ready to "vigilantly ensure" that the agreement is being implemented. Despite the nuclear diplomatic maneuvering, an adviser to Iran's supreme leader, Ayatollah Ali Khamenei, called on Friday for direct talks with the United States separate from the six-power negotiations. That appeared to signal top-level endorsement of the policies of President Hassan Rouhani in trying to reduce tensions. Ali Akbar Velayati told Iranian media that Tehran would benefit by talking separately with the U.S. and the other five powers because each nation has separate interests "over various international issues." Associated Press writer Deb Riechmann contributed from Washington.