Robert Malley, the State Department’s Iran envoy, said recently that while “it is in Iran’s hands to choose” which path to take, the United States and other allies need to be prepared for whichever choice Tehran makes.

He noted that Mr. Biden and Secretary of State Antony J. Blinken “have both said if diplomacy fails, we have other tools — and we will use other tools to prevent Iran from acquiring a nuclear weapon.”

But inside the White House, there has been a scramble in recent days to explore whether some kind of interim deal might be possible to freeze Iran’s production of more enriched uranium and its conversion of that fuel to metallic form — a necessary step in fabricating a warhead. In return, the United States might ease a limited number of sanctions. That would not solve the problem. But it might buy time for negotiations, while holding off Israeli threats to bomb Iranian facilities.

Buying time, perhaps lots of it, may prove essential. Many of Mr. Biden’s advisers are doubtful that introducing new sanctions on Iran’s leadership, its military or its oil trade — atop the 1,500 Mr. Trump imposed — would be any more successful than past efforts to pressure Iran to change course.

And more aggressive steps that were successful years ago may not yield the kind of results they have in mind. Inside the National Security Agency and U.S. Cyber Command, there is consensus that it is much harder now to pull off the kind of cyberattack that the United States and Israel conducted more than a decade ago, when a secret operation, code-named “Olympic Games,” crippled centrifuges at the Natanz nuclear enrichment site for more than a year.

Current and former American and Israeli officials note that the Iranians have since improved their defenses and built their own cyberforces, which the administration warned last week were increasingly active inside the United States.

The Iranians have also continued to bar inspectors from key sites, despite a series of agreements with Rafael M. Grossi, the head of the International Atomic Energy Agency, the United Nations’ watchdog, to preserve data from the agency’s sensors at key locations. The inspectors’ cameras and sensors that were destroyed in the plant explosion in late spring have not been replaced.