Updated

U.N. nuclear inspectors have counted nearly 200 advanced machines fully or partially installed at Iran's main uranium enrichment site, the International Atomic Energy Agency reported Thursday, confirming diplomats' accounts that Tehran has begun a major upgrade of a program that can be used in the making of atomic arms.

Iran denies it wants such weapons and says it is enriching only to make reactor fuel and for scientific and medical purposes under international law specifically allowing such activities. But because it hid its enrichment program -- and other nuclear activities -- for decades, many countries fear that it ultimately wants to enrich to weapons-grade level, suitable for arming nuclear warheads.

Iran announced last month that it planned to upgrade its Natanz enrichment facility, then said earlier this month that it had started doing so.

On Wednesday, diplomats told The Associated Press that upward of 100 enriching centrifuges had already been installed.

However, the IAEA report, circulated Thursday to the 35-nation agency board, was the first independent and on-record confirmation that the work had begun and was advancing. The confidential IAEA report, which was leaked to news media, said IAEA inspectors saw 180 of the high-tech IR 2m centrifuges fully or partially mounted at Natanz during a Feb. 6 inspection tour.

The advance is significant both in terms of technology and timing. The IR-2m centrifuges can enrich three to five times faster than the outmoded machines now being used at Natanz. For nations fearing that Iran may want to make nuclear arms that means a quicker way of getting there, should Tehran actually break out of its present peaceful enrichment program to openly work on a weapon.

The start of the upgrade is also of concern to six world powers that are preparing to open talks with Iran about its nuclear program on Tuesday in Kazakhstan. They want Tehran to cut back on enrichment, saying the installation of new machines instead sends the signal that the Islamic Republic is expanding that activity.