US Secretary of State Condoleezza Rice says she sees no need to get involved in European efforts to persuade Iran to drop its nuclear programme.
Iran agreed to halt enriching uranium last November
She was speaking on her way to London at the start of a tour which will see her visit a number of European countries and the Middle East.
European officials have repeatedly asked Washington for closer cooperation in dealing with the programme.
But Ms Rice said Tehran knew what it needed to do.
"It's not the absence of anybody's involvement that is keeping the Iranians from knowing what they need to do," she said.
"They need to live up to their obligations, they need to agree to verification inspection, they need to stop trying to hide activities under cover of civilian nuclear power."
Iran insists its nuclear programme is for purely peaceful purposes, but the US maintains Tehran wants to develop nuclear weapons.
Last November Tehran promised France, Germany and Britain that it would refrain from uranium enrichment and plutonium reprocessing for as long as talks go on.
But Washington wants the Europeans to take a tougher stance in trying to get Iran to give up the programme altogether.
Ms Rice also criticised Iran's human rights record.
"The Iranian regime's human rights behaviour and its behaviour toward its own population is something to be loathed," she said.
"I don't think anybody thinks that the unelected mullahs who run that regime are a good thing for the Iranian people and for the region," she added.