The United Nations’ chief nuclear inspector said on Friday that Iran was quadrupling its production of near-bomb-grade material, a move likely to intensify the challenge it will pose to the incoming Trump administration — but one that might also open the way to a new negotiation with the West.

“It is a dramatic acceleration,” Rafael M. Grossi, the director general of the International Atomic Energy Agency, said as the organization sent out a confidential assessment to its member states.

In a text message, he said that his inspectors had seen a quadrupling of production of uranium enriched to 60 percent purity, just shy of the levels needed to produce a weapon.

“These actions by Iran are worrying,” added Mr. Grossi, who visited Tehran last month and left the country in hopes that he had won its agreement to freeze new production of the fuel that has put Iran at the threshold of being able to produce a bomb.

But after the board of the I.A.E.A. passed a resolution that condemned Iran for blocking inspectors from key sites and dismantling some inspection gear, the Iranians responded by saying they would enrich uranium faster than ever before. And that appears to be exactly what they did at a plant called Fordow, built deep into a mountain so that Israel could not bomb it.

“Our inspectors are on the sites confirming that the process has indeed started,” Mr. Grossi said in a text exchange. “It is a very steep increase” that he said was still accelerating. “Now more than ever engagement and diplomacy are necessary.”

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