President Barack Obama's administration urged lawmakers Thursday not to impose new sanctions on Iran as it seeks to respond to overtures from new leader Hassan Rouhani.
"We have been clear that only concrete and verifiable steps can offer a path to sanctions relief," Wendy Sherman, undersecretary of state for political affairs, told lawmakers.
"Let me assure you that we will continue to vigorously enforce the sanctions that are in place as we explore a negotiated resolution, and will be especially focused on sanctions evasion and efforts by the Iranians to relieve the pressure."
International sanctions over what the West says is Tehran's nuclear weapons program have badly hit Iran's economy and its leaders have made it clear they are looking for relief.
But Sherman, speaking to the Senate Foreign Relations Committee, warned that the political crisis gripping Washington was hampering its ability to enforce the punitive measures.
"Our ability to do that, to enforce sanctions, to stop sanction evaders, is being hampered significantly by the shutdown," she said.
At midnight on Monday, the US federal government ran out of funding after a divided Congress failed to pass a stop-gap budget measure, sending hundreds of thousands of government workers homes and curtailing federal agencies' work.
The US Congress is debating whether to slap a fresh round of sanctions against Tehran, already placed under heavy punitive measures by the UN, United States and European Union.
Sherman is leading the US delegation in talks between Western negotiators and Iranian representatives.
She urged lawmakers to hold off on any new sanctions until after a new meeting set to take place later this month in Geneva, in a first test of Rouhani's recent overtures.
"In terms of legislation that is currently being discussed here on the Hill, we do believe it would be helpful for you all to at least allow this meeting to happen on the 15th and 16th of October before moving forward to consider those new sanctions," Sherman said.
Sherman called for "concrete, substantive actions" from Iran, as well as verifiable commitments, monitoring and verification.
"But I can assure you, if you do not come on the 15th and 16th with that substantive plan that is real and verifiable, our Congress will take action, and we will support them to do so," she added.
Turning to lawmakers, Sherman said she was hoping they would give the Obama administration "the time to begin these negotiations and see if, in fact, there is anything real here with my telling of the Iranians quite directly that if there isn't, that everyone is ready to act."
During an earlier visit to Tokyo, Secretary of State John Kerry said the United States will not take Iran at its word over pledges of openness on its nuclear program, after Israel threatened to act against Tehran.
The major powers and Israel suspect Iran is seeking to develop nuclear weapons under the guise of a civilian energy program.