U.S. Energy Secretary Steven Chu criticized Iran Monday for its nuclear activities, accusing the Islamic Republic of "denial, deceit and evasion.''
Iran has moved some of its uranium enrichment operations to an underground bunker. The West fears it is a first step in building a core for a nuclear warhead.
But Iranian Vice President Fereydoun Abbassi Davani blamed what he called Western "hostilities" for the move, saying Iran is trying to protect its program from outside intervention.
It is the second time in a week that the Obama administration has criticized Iran's nuclear aims.
The U.S. envoy to the International Atomic Energy Agency, Glyn Davies, said last week that Iran is "flouting" its obligations by continuing sensitive nuclear activities despite U.N. Security Council demands for a suspension of such work. He was speaking at a meeting of the U.N. nuclear agency's governing board in Vienna.
Davies also said a recent Iranian letter to European Union foreign policy chief Catherine Ashton does not contain any new Iranian commitments to address international concerns about possible Iranian experiments on nuclear warheads.
IAEA chief Yukiya Amano said last week he is "increasingly concerned" about new intelligence from agency members suggesting Iran is engaged in such experiments. Amano said Iran is showing "greater transparency" than usual about its nuclear program but is not providing "necessary cooperation" with the agency on nuclear matters.
European Union nations told an IAEA meeting last week that Amano's report to the agency shows Iran is "advancing in an extremely concerning direction."
Iran says its nuclear program is intended only to generate electricity for peaceful purposes.
Some information for this report was provided by AP, AFP and Reuters.