The United States on Monday appeared to dismiss a key Iranian demand to salvage a nuclear accord as Tehran blamed Washington for the prolonged impasse.
Iran and the United States have been negotiating indirectly in Vienna for a year to restore the 2015 agreement, known formally as the Joint Comprehensive Plan of Action, from which former US president Donald Trump withdrew.
One key sticking point is Iran's insistence on removing the US designation made by Trump that the Revolutionary Guards is a terrorist organization.
"If Iran wants sanctions lifting that goes beyond the JCPOA, they'll need to address concerns of ours that go beyond the JCPOA," State Department spokesman Ned Price said when asked about the Revolutionary Guards' blacklisting.
"If they do not want to use these talks to resolve other bilateral issues, then we are confident we can very quickly reach an understanding on the JCPOA and begin to reimplement the deal itself," Price told reporters.
President Joe Biden's administration has offered to return to the agreement, under which Iran was promised sanctions relief for curbing its nuclear program, but has voiced frustration at the slow pace of negotiations.
In Tehran, Iranian foreign ministry spokesman Saeed Khatibzadeh said that more than one issue was pending between Iran and the United States.
"Messages (from Washington) sent through (European Union coordinator Enrique) Mora these past weeks... are far from providing solutions that could lead to an accord," he told reporters.
"The United States are responsible for these delays, because they are taking their time to give replies" that would be suitable for Iran, AFP quoted him as saying.
Mora, who coordinates the indirect US-Iran talks, visited Tehran last month for talks with Iranian officials and later went to Washington, saying he hoped to close the gaps in negotiations.
Trump reimposed sweeping sanctions, including demanding other nations not buy Iran's oil, as he withdrew from the agreement negotiated by his predecessor Barack Obama.
Iran, in response, began rolling back on most of its commitments under the accord.