The United Nations nuclear agency says Iran is continuing to stockpile nuclear material and is preparing equipment to enrich uranium to higher levels.
A restricted report by the International Atomic Energy Agency says Iran's stockpile of low-enriched uranium has grown to more than 2.5 tons.
Iran recently brokered a deal with Brazil and Turkey to ship out 1.2 tons of its uranium in exchange for nuclear fuel. But, with such a large stockpile, Iran still would have enough uranium left over that after further enrichment, could be used to produce a nuclear weapon.
Iran insists its nuclear program is for peaceful purposes.
White House National Security Council spokesman Mike Hammer said the report underscores Iran's refusal to comply with the international requirements needed for negotiations on its nuclear program.
The IAEA says Iran is currently enriching uranium at close to 20 percent purity.
Iran said in February that it had raised its enrichment level to power a medical research reactor in Tehran. But the decision heightened Western concerns because it brings the material closer to the 90 percent needed to make nuclear weapons.
The IAEA report says the agency is still concerned about possible military dimensions to Iran's nuclear activities.
The five permanent members of the U.N. Security Council - the United States, France, Britain, China and Russia - have approved a draft resolution for a fourth round of U.N. sanctions against Iran because of its refusal to stop enriching uranium.
Some information for this report was provided by AP, AFP and Reuters.