The United States said Monday that a UN agency report showed "Iran's continued failure to comply with its international obligations and its sustained lack of cooperation" with the UN on its nuclear program.

National Security Council spokesman Michael Hammer reacted to portions of a restricted UN report which revealed on Monday that Iran had produced at least 5.7 kilos (12.5 pounds) of higher-enriched uranium.

The International Atomic Energy Agency (IAEA) said in the restricted report that it "remains concerned about the possible existence in Iran of past or current undisclosed nuclear related activities, involving military-related organizations."

"This latest IAEA report clearly shows Iran's continued failure to comply with its international obligations and its sustained lack of cooperation with the IAEA," Hammer said in a statement.

"Most notably, the report outlines Iran's continued uranium enrichment at both 3.5-percent and near-20-percent levels, construction of a heavy water research reactor, and refusal to permit the IAEA the access necessary to answer the ongoing questions... and long outstanding questions that surround a possible military dimension to its nuclear program."

Hammer added that the IAEA report "underscores that Iran has refused to take any of the steps required of it... which are necessary to enable constructive negotiations on the future of its nuclear program."

Iran, which has so far been enriching uranium to levels of no more than 5.0 percent in Natanz, started enriching to close to 20-percent purification in February, ostensibly to make fuel for the research reactor in Tehran.

The move drew wide condemnation from western countries because it brings the Islamic republic closer to levels needed to make the fissile material for a nuclear bomb.

In an IAEA-brokered deal last October, the United States, Russia and France proposed they take most of Iran's stockpile of low-enriched uranium (LEU) and turn it into the fuel rods for the research reactor.

But Iran refused to take up the offer and has drawn up an alternative deal with Brazil and Turkey.

And it has riled the West by insisting on enriching uranium to higher levels on its own, even though it is not believed to have the technology to turn that material into the fuel rods for the reactor.