Iran on Wednesday announced an upgrade to its uranium enrichment machines, upping the ante even as UN experts were holding talks in Tehran on the Islamic republic's controversial nuclear drive.

The announcement comes despite a warning by Washington that an Iranian upgrade would violate UN resolutions and claims by Israeli Prime Minister Benjamin Netanyahu that Tehran is moving closer to making a nuclear bomb.

Atomic Energy Organisation chief Fereydoon Abbasi Davani said new centrifuges with "a higher efficiency" were being installed at Iran's Natanz site, which uses the machines to enrich uranium gas by spinning it at supersonic speeds.

"The installation of new centrifuges at Natanz site started about a month ago... the process is ongoing," Abbasi Davani said in his announcement, which was reported by local media.

He did not reveal how many centrifuges were installed but said: "These centrifuges have a higher efficiency and are used specifically to enrich uranium to less than 5 percent, and not to 20 percent" -- the purity regarded as being a short step from that which is required to make a nuclear bomb.

Iran had indicated to the UN nuclear watchdog its intention to install the new generation equipment, according to a document seen by AFP at the end of January.

The International Atomic Energy Agency document said that Iran informed it in a letter dated January 23 that "centrifuge machines type IR2m will be used in Unit A-22" at the Fuel Enrichment Plant at Natanz.

The IAEA replied in a letter dated January 29 asking for more information on the announcement.

Two days later White House spokesman Jay Carney warned that Iran's plans to install more modern equipment at Natanz was a "further escalation" in the showdown over its atomic programme.

"The installation of new advanced centrifuges is a further escalation and a continuing violation ... of Iran's obligations under relevant United Nations Security Council and IAEA board resolutions," Carney said.

Netanyahu warned on Monday that Iran was now closer to crossing the "red line" after which it would be able to build a nuclear weapon. He also asked for "stronger pressure and harsher sanctions" against Tehran.

Abbasi Davani's statement Wednesday on the centrifuge update came as the UN experts were locked in talks in Tehran with a nuclear team led by Iran's IAEA envoy Ali Asghar Soltanieh.

The visit by the IAEA inspectors is aimed at addressing international fears of a possible military dimension to Iran's nuclear drive and comes ahead of a new round of talks between Iran and six world powers in parallel diplomatic efforts in Kazakhstan on February 26.

The goal of the meeting in Tehran, the third of its kind in the past three months, is to finalise a "structured approach document", according to Herman Nackaerts, the IAEA's chief inspector who is leading the delegation to Tehran.

The document would "facilitate the resolution of the outstanding issues related to the possible military dimension of Iran's nuclear programme," Nackaerts told journalists before leaving Vienna on Tuesday.

But he said "differences remain" and that his delegation would work hard to resolve them. "We will have good negotiations."

The Vienna-based agency says "overall, credible" evidence exist that until 2003, and possibly since, Iran conducted nuclear weapons research.

Vehemently rejecting the charges, Iran has denied the IAEA broader access to sites, scientists and documents involved in these alleged military activities.

The IAEA is hoping to gain access to Parchin, a military base near Tehran where the agency suspects Iran could have carried out experiments with explosives capable of triggering a nuclear weapon.

But Abbasi Davani on Wednesday dented the prospects of such visit.

"A visit to Parchin or any other site is not on the agenda," he said, according to ISNA news agency.

"We will negotiate and hope to reach a rational framework," he said.

Previous demands by the IAEA to visit the base were spurned by Iran, which insists agency inspectors already visited Parchin twice in 2005 and found nothing untoward.

But the agency counters that activity at Parchin spotted by satellite, including moving "considerable" volumes of earth, makes it want to go back.

Iran has been slapped with multiple sets of UN Security Council sanctions for its refusal to stop uranium enrichment. The United States and the European Union have also imposed their own additional sanctions.