The Obama administration said Thursday it would support tougher economic pressure on Iran if the Islamic republic doesn’t begin slowing the pace of its uranium enrichment activity and opening its stockpiles of nuclear material to greater inspection, and reassured its critics that the U.S. would not be played for “suckers” by the moderate tone of Iran’s new leader.

The chief U.S. nuclear negotiator told the Senate Foreign Relations Committee that the administration could offer the Iranians some sanctions relief as “confidence-building” measures but that it would support new and tougher trade restrictions from Congress if diplomacy ultimately fails to ease concerns that Iran might be trying to develop nuclear weapons.

“I’m saying this” to Iran, said negotiator Wendy Sherman, who will meet with other world powers and Iran in Geneva in two weeks. “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.”

Speaking in Tokyo, Secretary of State John Kerry told reporters the U.S. would not be played for “suckers” by Iranian President Hassan Rouhani. Still, Kerry defended President Barack Obama’s recent engagement effort.

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. The administration had expressed concern about the sanctions undercutting Rouhani with hardliners in his own country or weakening the international consensus on Iran, given that China, Turkey, India and several other Asian countries still purchase oil from Tehran.

Sherman asked senators, however, to wait until after the Geneva talks before moving forward.

Kerry, responding to Israeli Prime Minister Benjamin Netanyahu’s urging not to trust Iran, defended the recent engagement effort. Kerry met last week at the United Nations with Iranian Foreign Minister Mohammad Javad Zarif, and then Obama placed a historic phone call to Rouhani — the first between U.S. and Iranian leaders in more than three decades.

He said it would be “diplomatic malpractice of the worst order not to test at least Iran’s rhetorical promises that it is prepared to negotiate. After a month during which Obama, Congress and the American people waivered on attacking Syria after a series of chemical weapons attacks there, Kerry stressed the importance of examining “every possibility” to avoid military action.

However, he added, “there is nothing here that is going to be taken at face value, and we have made that clear.”

“It is not words that will make a difference. It’s actions,” Kerry said.

Netanyahu has regularly threatened Iran with a possible Israeli strike and disparaged Rouhani, a former chief Iranian nuclear negotiators, as a “wolf in sheep’s clothing.” The Israeli leader met this week with Obama at the White House before delivering an address to the United Nations focused on the Iranian threat.

For the last several years, the U.S. has tried to reinforce its own threat of military force against Iran alongside increasingly devastating sanctions on the Iranian economy and promises of diplomatic engagement if Tehran shows greater flexibility in nuclear talks. While engagement has been given new life with the overtures from Kerry and Obama, the administration is in some ways of two minds on sanctions right now.

Sherman said the administration’s position would be determined significantly by the Geneva talks.

“We will be looking for specific steps by Iran that address core issues, including but not limited to the pace and scope of its enrichment program, the transparency of its overall nuclear program and stockpiles of enriched uranium,” Sherman said. “The Iranians, in return, will doubtless be seeking some relief from the comprehensive international sanctions that are now in place.”

Sherman said the U.S. would consider softening some sanctions if Iran imposes “some freeze, some pause” in its uranium and plutonium activity, which Iran insists is for peaceful energy production but which the U.S. and many other countries suspect is aimed at achieving nuclear weapons capability. However, she added that America’s “fundamental large sanctions” would remain in place until “all of our concerns are addressed” and that the administration would welcome additional sanctions from Congress if Iran doesn’t quickly outline a plan to alleviate international concerns.

Speaking to senators now in the third day of a government shutdown, Sherman also cautioned that gridlock was hampering the administration’s ability to enforce existing U.S. sanctions on Iran. She said the Treasury Department’s sanctions division has been “utterly depleted” and that the intelligence community is facing severe staffing reductions.

“We will do our best to enforce these sanctions, to stop sanctions evaders, but I sincerely hope that the shutdown ends soon,” she said.

(AP)