President Barack Obama's administration has urged lawmakers 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 is hampering its ability to enforce the punitive measures.
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"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.
