WASHINGTON — International inspectors who gained access to Iran's newly revealed underground nuclear enrichment plant voiced strong suspicions in a report on Monday that the country was concealing other atomic facilities.

The report was the first independent account of what was contained in the once-secret plant, tunneled into the side of a mountain, and came as the Obama administration was expressing growing impatience with Iran's slow response in nuclear negotiations.

In unusually tough language, the International Atomic Energy Agency appeared highly skeptical that Iran would have built the enrichment plant without also constructing a variety of other facilities that would give it an alternative way to produce nuclear fuel if its main centers were bombed.

So far, Iran has denied that it built other hidden sites in addition to the one deep underground on a military base about 12 miles north of the holy city of Qum. The inspectors were given access to the plant late last month and reported that they had found it in "an advanced state" of construction, but that no centrifuges — the fast-spinning machines needed to make nuclear fuel — had yet been installed.

The inspectors said Iran had "provided access to all areas of the facility" and planned to complete it by 2011. They also confirmed U.S. and European intelligence reports that the site had been built to house about 3,000 centrifuges, enough to produce enough material for one or two nuclear weapons a year. But that is too small to be useful in the production of fuel for civilian nuclear power, which is what Iran insists is the intended purpose of the site.

The report comes just two days after President Barack Obama, on a trip to Asia, said "we are running out of time" for Iran to sign onto a deal to ship part of its nuclear fuel out of the country. He said he would begin to plan for far more stringent economic sanctions against Tehran.

He was joined during that announcement by President Dmitri A. Medvedev of Russia, but Medvedev was vague about whether Russia was prepared to join in those sanctions. Obama was expected to take up the issue on Tuesday with President Hu Jintao of China, where Obama is on a state visit. China, like Russia, has historically resisted sanctions on Iran.