Washington’s chief nuclear negotiator says the government shutdown in the United States is emptying offices charged with enforcing sanctions on Iran and “significantly” hindering America’s ability to enforce its sanctions regime.

“Our ability to enforce sanctions and stop sanctions evaders is being hampered significantly by the shutdown,” Wendy Sherman, the US undersecretary of state for political affairs, told the Senate Foreign Relations Committee in a briefing on Iran Thursday. “Government shutdown empties offices enforcing sanctions on Iran.”

Such offices in the Department of the Treasury and the State Department have been “utterly depleted,” she said. “In addition, the intelligence community, which we rely on for intelligence information to go after sanctions evaders and people who are not paying attention to the sanctions… has been devastated as well.”

President Barack Obama canceled a trip Thursday to Asia to stay in Washington and push for an elusive funding bill to end the federal government shutdown, which entered its fourth day on Friday.

In her comments Thursday, Sherman said that Israel’s security remains a “paramount focus” as the international community sounds out Iran’s intentions regarding its nuclear program. She asked Congress to give the administration more time to pursue the diplomatic track with the Islamic Republic before further decisions on sanctions are made.

The Senate Banking Committee is expected to draft a new sanctions package later this month, mirroring legislation passed by the House in July that blacklists Iran’s mining and construction sectors and commits the United States to the goal of eliminating all Iranian petroleum sales worldwide by 2015.

“It will be the Iranian government’s actions in the months ahead that will be a key factor in determining whether we decide the sanctions should remain in place, or whether we can begin to relieve some sanction pressure as Iran addresses our concern,” she said.

“I’m saying this” to Iran, said Sherman, who will meet with other world powers and Iran in Geneva in two weeks’ time. “Come on the 15th of October with concrete, substantive actions that you will take, commitments you will make in a verifiable way, monitoring and verification that you will sign up to, to create some faith that there is reality to this, and our Congress will listen. 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.

“We will remain in close consultations with our allies and partners in the region, including Israel,” she continued.

Officials in Jerusalem are concerned that Iran is biding time with false offers to the West while it progresses toward a nuclear weapons capability. Iran insists that its nuclear program is peaceful in nature.

Iranian President Hassan Rouhani says he has a mandate to pursue an agreement that satisfies the international community’s concerns over Iran’s nuclear program, said Sherman, promising that the US is “prepared to test that proposition in a serious way.”

At the UN last week, Rouhani delivered a conciliatory speech in which he said Iran has no intention of building a nuclear weapon and declared his readiness for new negotiations with world powers. But he stressed that Iran would not halt what he said was a peaceful nuclear program.

Capping off the visit, Rouhani and Obama held a 15-minute phone call as the Iranian leader was traveling to the airport. It was the first conversation between the nations’ leaders in 34 years and raised hopes that a breakthrough on the nuclear issue could portend even deeper ties between the US and Iran.

But Sherman promised that the administration would not be fooled by Iran. “We must remain mindful of the long history of Iranian deception regarding its nuclear program,” she emphasized, echoing earlier comments by US Secretary of State John Kerry. While he defended Obama’s recent engagement effort, Kerry told reporters in Tokyo that the US would not be played for “suckers” by Rouhani.

Sherman said that, moving forward, the United States would look for new, concrete, and verifiable actions. She indicated that the US would seek confidence-building steps from the Iranian government on both the pace and transparency of the nuclear program.

Sherman also gave details of Kerry’s meeting with Iranian Foreign Minister Mohammad Javad Zarif on the sidelines of last week’s United Nations General Assembly in New York. She characterized the meeting as the beginning of the process of testing Iran’s intentions, and called Zarif’s presentation “thoughtful.” Zarif insisted that Iran does not seek nuclear weapons, and presented a number of reasons why it did not make sense for Iran to possess them.

The Associated Press contributed to this report.