A U.S. official told Congress Thursday there is no evidence Iran is giving up what Washington believes is a nuclear weapons program.
The comments by undersecretary of State for political affairs, Nicholas Burns, to the Senate Foreign Relations Committee come as European and Iranian negotiators prepare to discuss the issue Tuesday.
"We see no sign that Iran has made the necessary strategic decision to abandon its nuclear ambitions," said Mr. Burns.
The foreign ministers of France, Britain and Germany, along with EU foreign policy chief Javier Solana, are to meet with Iranian officials, most likely in Brussels.
The three European countries called the meeting after Iran announced it would resume uranium conversion work, a move that would violate a November accord on freezing nuclear fuel work.
Mr. Burns says U.S. allies have given assurances they would deny Iran's nuclear ambitions and would seek consequences if Tehran violated the November agreement.
"Our European partners made clear that Iran must provide objective guarantees to demonstrate that it is not pursuing a clandestine weapons program under the cover of a civilian nuclear energy program," he added. "On this point the bar for Iran must be set very high because of its history of deception, which has undermined the trust of the international community."
Iran has agreed to suspend its nuclear activities, which it says are only aimed at generating electricity, pending the outcome of next week's talks.
Mr. Burns had a message for Iran going into the negotiations:
"Adhere to the Paris agreement, maintain suspension of all nuclear-related activities, and negotiate in good faith the eventual cessation and dismantling of all sensitive nuclear fuel cycle activities," he said.
France, Germany and Britain say if Iran resumes uranium enrichment, they will support the United States in seeking U.N. Security Council action against Tehran.
In the U.S. House of Representatives, Congressman Jim Saxton, a New Jersey Republican, introduced legislation calling for Security Council action against Iran if it resumes its nuclear program.