AP Photo

Iran told the United Nations nuclear watchdog on Thursday that it would not accept a plan its negotiators agreed to last week to send its stockpile of uranium out of the country, according to diplomats in Europe and American officials briefed on Iran’s response.

The apparent rejection of the deal could unwind President Obama’s effort to buy time to resolve the nuclear standoff.

In public, neither the Iranians nor the watchdog, the International Atomic Energy Agency, revealed the details of Iran’s objections, which came only hours after Iran’s president, Mahmoud Ahmadinejad, insisted that “we are ready to cooperate” with the West.

But the European and American officials said that Iranian officials had refused to go along with the central feature of the...