BRUSSELS — The United States on Wednesday accused Iran of “nuclear extortion” and threatened further sanctions against Tehran, which has begun stockpiling and enriching uranium beyond the limits set in the 2015 accord that President Trump has abandoned.

The United States called an emergency meeting of the International Atomic Energy Agency in Vienna on Wednesday in response to the Iranian moves, while a senior French envoy was in Tehran exploring ways to reopen negotiations on compliance with the deal.

Both Iran and the United States insist that they are open to further negotiations, but both have put different conditions on new talks. Washington insists that they must prevent Iran’s development of a weapons program and restrict Iran’s involvement with regional allies, while Iran insists that Washington must first rejoin the deal Mr. Trump renounced in 2018 and remove sanctions that are strangling Iranian oil exports.

At the Vienna meeting, the American ambassador, Jackie Wolcott, said that “there is no credible reason for Iran to expand its nuclear program, and there is no way to read this as anything other than a crude and transparent attempt to extort payments from the international community.” The only path to sanctions relief, she said, is “through negotiations, not nuclear extortion.”