WASHINGTON — Three weeks ago, in a show of both good faith and diplomatic pressure, the United States offered to rejoin nuclear talks with Iran. The double-edged overture fell flat: Iran refused to meet without first receiving financial incentives, and the Biden administration made clear, as the White House national security adviser, Jake Sullivan, put it, that “the ball is in their court.”

That set off a new rush among world powers to resuscitate a 2015 nuclear accord that the United States exited three years after negotiating it, leaving Iran to steadily violate the terms of the deal.

Diplomats from Britain, France and Germany have since urged Iran to accept a joint European-American invitation on Feb. 18 to begin informal negotiations. Officials from China and Russia have taken a more sympathetic approach in asking Tehran in recent days to return to talks. President Hassan Rouhani of Iran discussed the delicate diplomacy in a phone call with Prime Minister Boris Johnson of Britain on Wednesday and President Emmanuel Macron of France last week.

“We have to use this window of opportunity,” Josep Borrell Fontelles, the European Union’s top foreign policy official, told an Atlantic Council forum on Feb. 23.

Mr. Grossi kept Tehran from crossing a diplomatic red line.

it withdrew from the deal in May 2018.

“America was first in breaking with the agreement and it should be the first to return to it,” Mr. Rouhani said on Wednesday during a cabinet meeting in Tehran.

However, he added: “America should know that we are ready to implement the agreement. We are ready to implement it full in return for full and parts in return for parts. We are ready to return to our full commitments for their full return or part of our commitments for their partial return.”

wait-and-see approach toward negotiations.

He appears torn between allies in Europe and critics in Congress over broadening the nuclear accord to also limit Iran’s ballistic missiles program and its support for proxy militias across the Middle East.

Though many senior administration officials had negotiated the nuclear deal while working for President Barack Obama, and still support it, they also say they are unwilling to compromise further — particularly as Iran persistently tests Mr. Biden’s limits.

“Can you assure us that we’re not going to make concessions just to get a meeting?” Representative Brad Sherman, Democrat of California, asked Secretary of State Antony J. Blinken on Wednesday during a House hearing, referring to the nuclear accord, known as the Joint Comprehensive Plan of Action.

“I can,” Mr. Blinken responded.

“Do we expect that before we give them sanctions relief that they will verifiably either be in full compliance with the J.C.P.O.A. or be on a negotiated path toward full compliance?” Mr. Sherman asked.

“Yes,” Mr. Blinken said.

The demands for a broader accord to address other Iranian threats echoes the Trump administration’s goals of a pressure campaign against Tehran. But Mr. Biden’s pursuit to “lengthen and strengthen” the deal is also calculated to assuage Democratic critics of the 2015 accord.

Among them is Senator Bob Menendez, Democrat of New Jersey and chairman of the Foreign Relations Committee, who oversees the State Department and the approval process for presidential nominees to work there.

“Iran’s continuous engagement on so many other fronts — on the ballistic missiles, on the destabilization of the region, on its continuing advocacy for terrorism to its proxies — you know that just going back to the J.C.P.O.A. is really a difficult proposition,” Mr. Menendez told reporters in Washington on Tuesday.