The nuclear impasse with Iran is also on the G20 agenda.

In the vortex of global challenges facing the Group of 20 leaders — the tenacious coronavirus, the disrupted economy, the warming climate — lurks a less prominent but no less vexing issue: what to do about Iran’s expanding nuclear program.

President Biden and his counterparts are certain to discuss efforts aimed at reviving the 2015 nuclear accord with Iran, which Mr. Biden’s predecessor, Donald J. Trump, abandoned in 2018, calling it insufficiently strict.

Under the agreement, Iran sharply curtailed its nuclear activities in verifiable ways, aimed at ensuring that it could not make an atomic bomb, and the United States rescinded some sanctions that had severely crimped Iran’s economy.

Since the U.S. repudiation of the agreement, and the restoration of sanctions, Iran is no longer abiding by its terms, either. According to U.N. monitoring reports, Iran has made significant advances in enriching uranium, the nuclear fuel that can be used for both peaceful pursuits and for weapons. It now has far more enriched uranium than it did in 2018, and has enriched it closer to the very high level needed to make a bomb.

Although Iran has repeatedly pledged that it will never become a nuclear-weapons state, it is believed to be close to crossing an important threshold, having amassed roughly enough uranium for fueling a bomb.

Mr. Biden has said he wants to restore U.S. participation in the agreement. The other parties to the accord — Britain, China, France, Germany and Russia — have been seeking ways to save it.

But talks with Iran on this issue have basically been stalled since the June election of Iran’s hard-line president, Ebrahim Raisi, who has insisted that the United States return to compliance first, promise to never abandon the accord again and give up any thought of renegotiating its terms.

Biden administration officials have suggested that time is running out to salvage the agreement.

On Wednesday, Iran’s deputy foreign minister said that Iran intended to participate in talks in Vienna on reviving the accord before Nov. 30, but as of this weekend a date had not been set.

Outside experts who have followed the ups and downs of the accord’s history have turned increasingly skeptical about the prospects for saving it.

“Iran’s continued intransigence and the acceleration of its nuclear program will make it difficult for even the most forward-leaning negotiators to revive the agreement next year,” the Eurasia Group, a geopolitical risk advisory firm, said this past week in an assessment written by its Iran analysts.