US Secretary of State John Kerry urged lawmakers on Tuesday not to impose news sanctions on Iran, adding that such measures would hamper efforts to forge a long-term deal on its disputed nuclear programme, dpa reported.
Kerry defended a deal reached in Geneva last month that calls on Iran to limit some of its nuclear activities in exchange for partially easing crippling economic sanctions.
"We will be the first ones to come to you if this fails and ask you for additional sanctions," Kerry told the House Foreign Affairs Committee.
Kerry said the United States would struggle to convince other world powers to uphold the agreement if Congress imposed new sanctions, describing talks with Iran as a "very delicate diplomatic moment."
He said further sanctions could endanger the effectiveness of existing sanctions and could give Iran an excuse to flout the agreement.
Kerry stressed the sanctions had brought Iran to the negotiating table and that time was needed to say whether Iran had "changed its nuclear calculus."
US lawmakers have continued to push for tougher sanctions despite the Geneva agreement.
"Iran is not just another country, it simply cannot be trusted with enrichment technology," said Republican Congressman Ed Royce, who chairs the committee.