VIENNA: Iran has told the U.N. nuclear agency that it is running a new, previously undeclared, facility to enrich uranium, officials told The Associated Press Friday.

Iran is under three sets of U.N. Security Council sanctions for refusing to freeze enrichment. The officials said that Iran revealed the existence of a second enrichment plant in a letter sent Monday to International Atomic Energy Agency chief Mohamed ElBaradei.

It had previously said it was operating only one plant, which is being monitored by the IAEA.

The Islamic Republic insists that it has the right to the activity to generate fuel for what it says will be a nationwide chain of nuclear reactors.

But because enrichment can make both nuclear fuel and weapons-grade uranium, the international community fears Tehran will use the technology to generate the fissile material used on the tip of nuclear warheads.

The revelation further burdens the chances of progress in scheduled Oct. 1 talks between Iran and six world powers.

At that planned meeting the first in more than a year the five permanent U.N. Security Council members and Germany will be pressing Iran to scale back on its enrichment activities. But Tehran has declared that it will not bargain on enrichment.

The officials said that the letter contained no details about the location of the second facility, when it had started operations or the type and number of centrifuges it was running.

The government officials one speaking from his European capital outside Vienna, the other a diplomat in Vienna from a country accredited to the IAEA demanded anonymity Friday because their information was confidential. One said he had seen the letter. The other told the AP that he had been informed about it by a U.N. official who had seen it.

While Iran's mainstay P-1 centrifuge is a decades-old model based on Chinese technology, it has begun experimenting with state-of-the art prototypes that enrich more quickly and efficiently than its old model.

U.N. officials familiar with the IAEA's attempts to monitor and probe Iran's nuclear activities have previously told the AP that they suspected Iran might be running undeclared enrichment plants.

The existence of a secret Iranian enrichment program built on black market technology was revealed seven years ago. Since then the country has continued to expand the program with only a few interruptions as it works toward its aspirations of a 50,000-centrifuge enrichment facility at the southern city of Natanz.

The last IAEA report on Iran in August said Iran had set up more than 8,000 centrifuges to churn out enriched uranium at the cavernous underground Natanz facility, although the report said that only about 4,600 of those were fully active.