(CNN) -- Iran has stepped up efforts to produce enriched uranium in violation of international resolutions to stop, the International Atomic Energy Agency said Friday in a report posted online by a nonproliferation group.
In the report, published by the Institute for Science and International Security, a nonprofit group focused on nuclear nonproliferation efforts, the IAEA said Iran is not cooperating with efforts to verify that its nuclear program is purely peaceful, leaving the agency with "serious concerns" about potential military uses.
Iranian officials refused to provide IAEA inspectors access to a key military installation during a visit that ended Tuesday and dismissed the concerns of inspectors as based on "unfounded allegations," according to the report, which has not yet been released by the IAEA.
But investigators have confirmed that Iran has expanded its capacity to enrich uranium, according to the report.
Enrichment involves increasing the concentration of uranium-235 from its natural concentration of less than 1% to the higher concentrations necessary to fuel nuclear power plants or other uses.
Nuclear power requires 3% to 5%. Weapons-grade uranium is enriched to about 90%. Iran is enriching some uranium to 20%, despite United Nations and IAEA resolutions to stop, the report published Friday confirmed.
While Iran has said the higher-level enrichment is meant to produce therapies for cancer patients, international critics have called the efforts a troubling step toward possible militarization of nuclear technology there.
IAEA inspectors left Iran after two days of talks failed to reach an agreement on how to verify Tehran's claim that its nuclear program is only for civilian purposes and after inspectors said they were denied an opportunity to visit a military base in Parchin.
The IAEA reported in November that Iran built a chamber at Parchin in 2000 that was designed to contain the force of up to 70 kilograms (154 pounds) of high explosives. Inspectors visited the site twice in 2005, but did not go to the building now believed to have housed the test chamber, that report stated.
Parchin may have been the site of tests of high explosives that could be used to detonate a nuclear bomb -- experiments the agency called "strong indicators of possible weapon development," it read.
The IAEA found that between the November report and its current one, Iran added 3,000 centrifuges for a total of 9,000 at the Natanz facility in central Iran, according to Paul Brannan, a senior analyst at the Institute for Science and Security. More centrifuges mean more enrichment capability.
"That's a significant jump," he said.
Brannan described Iran's growing stockpile of the enriched uranium as a "major concern" and questioned why so much is needed.
"They stockpiled so much of it they can operate for over a decade," Brannan told CNN.
The 20% level is significant because nuclear experts believe further enrichment needed to create a material for a possible weapon is easy to achieve once one is capable of 20%.
But Brannan said Iran is likely to be deterred from increasing the enrichment because of international attention, noting that it would have a hard time not arousing suspicions of inspectors if it tried to keep them from the enrichment sites in the time it would take to further enrich.
Another analyst, however, played down the significance of the latest IAEA report. Joel Rubin with the U.S.-based Ploughshares Fund, a group devoted to seeking the elimination of nuclear weapons, said it is "not a game changer."
Rubin said there are still valid concerns about Iran's nuclear ambitions and program, but there is no indication the country has achieved a new "breakthrough."
"It's an update that -- yes -- Iran is doing things that we are not fully read into and we need to understand," Rubin said. "But Iran has not moved to a more advanced program."
Iran's representative to the IAEA, Ambassador Ali Asghar Soltanieh, denied Friday that officials blocked inspectors from visiting Parchin, saying inspectors left a day early.
He told CNN that access to the complex would be granted once Iranian officials and the agency can agree on the conditions under which such a visit would take place.
"We are trying to be cooperative," he said.
Soltanieh described the talks with IAEA inspectors as "intense and constructive."
In a statement, IAEA officials agreed the discussions were intensive, but IAEA Director General Yukiya Amano called the lack of results from the visit "disappointing."
Iran is under intense international pressure to demonstrate that it has no intention of pursuing nuclear weapons, with widespread Western sanctions targeting its financial and oil sectors. Crude oil sales make up about half of Tehran's revenue.
"We call upon Iran to come into full compliance with U.N. Security Council resolutions by suspending its enrichment program and providing full and genuine transparency to the IAEA. If it refuses to shift course, its isolation from the international community will only continue to grow," National Security Council spokesman Tommy Vietor said in a statement.
CNN's Adam Levine and Stacia Deshisku contributed to this report.