The US secretary of state, John Kerry, has said that new sanctions being prepared by Congress would "break apart" ongoing negotiations with Iran about its nuclear programme.
Kerry was speaking on Wednesday before meeting US politicians about their plans for more sanctions against Tehran, whose economy has already been badly hit by international measures.
"The risk is that if Congress were to unilaterally move to raise sanctions it could break faith in those negotiations and actually stop them and break them apart," said ahead of the meeting with senators.
The State Department spokesman, Jen Psaki, said Kerry wanted a "temporary pause" on new sanctions to allow diplomats from six world powers to negotiate with Iran and to test whether it may be possible to resolve a 10-year standoff.
"The secretary will be clear that putting new sanctions in place would be a mistake while we are still determining if there
is a diplomatic path forward," Psaki said.
Members of both houses of Congress are currently debating a new round of sanctions on Tehran, more severe than those already in place that have destroyed import and export markets, increased unemployment and sent inflation rocketing.
No 'radical' change
Meanwhile, the leader of the UN's nuclear authority says Iran has "quite a lot to do" to complete a nuclear reactor which is of deep concern to the West and which formed a central sticking point in negotiations to lift sanctions.
Yukiya Amano, the director general of the International Atomic Energy Agency, also told the Reuters news agency on Wednesday that he had seen no "radical" change in Iran's nuclear activities since Hassan Rouhani became president three months ago.
Amano said Iran still had "quite a lot to do" in order to complete the Arak research reactor, a plant which is of deep concern to the West as it can produce potential bomb material once it is operating.
The heavy water reactor produces plutonium as a byproduct - a substance that has few civilian applications and can be used to build a nuclear bomb. Iran says the reactor will produce isotopes for medical and civilian purposes.
He added Tehran was continuing its most sensitive nuclear activity - enrichment of uranium to a fissile concentration of 20 percent.
Iran insists its nuclear programme is for civilian use but Western nations, suspecting Tehran is trying to develop a nuclear bomb, have imposed crippling sanctions.
The US and its allies narrowed their differences with Tehran after three days of intense diplomatic efforts in Geneva but failed to produce a breakthrough in the decade-long dispute.