Indirect U.S. negotiations with Iran over reviving the 2015 nuclear deal cannot go on “indefinitely”without success, a senior State Department official said following the completion in Vienna of a sixth round of talks.

“We’ve made progress” and “the Iranians have been serious,” the official said in a briefing for reporters Thursday. “But we do have differences, and if we can’t bridge them in the foreseeable future, we’re going to have to regroup.”

Dismissing reports that a framework agreement had been reached, the official repeated the administration’s mantra that “nothing is agreed until it is all agreed. . . . We are still negotiating every issue,” including the extent of U.S. sanctions relief, a rollback of Iranian nuclear activities in violation of the terms of the initial deal, and the sequencing of any steps that may be agreed.

“We still don’t have anything nailed down,” the official said. But negotiators would not return to Vienna for a seventh round of talks, as expected, “if we didn’t think it remained possible.”

The official, who spoke on the condition of anonymity under rules set by the State Department, said the Biden administration did not believe that the election of a new, hard-line Iranian president last week would affect Tehran’s negotiating position. Decisions are made by Iran’s supreme leader, Ali Khamenei, and he agreed to the negotiations, the official said.

Under the 2015 Joint Comprehensive Plan of Action, known as the JCPOA, the Obama administration lifted sanctions on Iranian trade and oil and Iran’s frozen overseas assets. Iran agreed to strict limits on its nuclear program that tightly restricted the quantity and quality of enriched uranium it could produce and possess, and to monitoring by the International Atomic Energy Agency.

While the agreement was in effect, the amount of time the U.S. government estimated it would take for Iran to produce enough fissile material for a nuclear weapon expanded from no more than a few months to about one year. After President Donald Trump withdrew from the agreement in 2018, reimposed the lifted sanctions and added more than 1,500 new ones in an effort to cripple Iran’s economy, Iran expanded its enrichment efforts. The breakout time is now estimated to be two to three months.

President Biden said during his campaign he would rejoin the deal, to which Britain, France, Germany, Russia and China are also signatories. The Europeans are acting as go-betweens in the Vienna talks, which began in April, since the Iranians refuse to speak directly to the United States.

Beyond remaining disagreements over Iran’s nuclear reduction commitments, and which U.S. sanctions are nuclear-related and eligible to be lifted to return the parties to the terms of the original deal, major impediments remain.

Iran is seeking assurances that a future U.S. administration will not again renege on the deal, a promise that American negotiators have said is impossible to provide. For its part, the United States wants Iran to explicitly agree to lengthen and strengthen the original terms of the JCPOA, and to open discussions of its ballistic missile program and support for proxy forces fighting in other countries. Iranian leaders have said publicly they will not do either of those things.

The senior official also said that Iran’s failure so far to reach an extension of a monitoring agreement with the IAEA, which expired on Thursday, could also pose a significant problem in returning to Vienna.