A Russian ship left Iran on Monday carrying almost all of Iran's stockpile of low-enriched uranium, fulfilling a major step in the nuclear deal struck last summer and, for the first time in nearly a decade, apparently leaving Iran with too little fuel to manufacture a nuclear weapon.
The shipment was announced by Secretary of State John Kerry and confirmed by a spokesman for Russia's civilian nuclear company, Rosatom. Mr. Kerry called it "one of the most significant steps Iran has taken toward fulfilling its commitment," and American officials say that it may be only weeks before the deal reached in July takes effect.
On "implementation day," roughly $100 billion in Iranian assets will be unfrozen, and the country will be free to sell oil on world markets and operate in the world financial system.
For President Obama, the peaceful removal of the fuel from Iran is one of the biggest achievements in his foreign policy record, the culmination of a seven-year effort that at various times involved sanctions, cybersabotage of Iran's main nuclear facility and repeated Israeli threats to bomb the country's facilities.
Less than a year ago, many inside the Obama administration — and almost all senior officials in Israel, which regards Iran as a dangerous foe — said they doubted Iran would agree to part with a stockpile of fuel that gave it the potential power to build a weapon, even though the Iranians have said that was not their intention.
Mr. Kerry, in a statement, said the ship, which Russian officials said was the Mikhail Dudin, carried 25,000 pounds of nuclear material. That included, Mr. Kerry said, the fuel that was closest to bomb-grade quality: It had been enriched to 20 percent purity. Iranian officials said that fuel was for a specialty reactor to make medical isotopes, but it was considered a threat because it would require relatively little further enrichment to produce a weapon.
Ridding Iran of the material was a major goal of the multistep agreement to unravel what the United States and international regulators called a military endeavor in the guise of a civilian nuclear program.
Iran is still disassembling centrifuges, which enrich uranium, and disabling a plutonium reactor, among other steps that are required under the nuclear agreement struck in July.
For face-saving purposes, Iran is calling the uranium shipment part of a "fuel swap." But the fuel it is receiving, partly from Kazakhstan, is natural uranium, which would require substantial processing to be used for either a nuclear reactor or a weapon.
Mr. Kerry's statement said that with the removal of the fuel, Iran's "breakout time" — the time needed to produce a weapon — had already moved from two to three months to six to nine months. Before the deal goes into effect, that time is supposed to extend to a full year.
Iran is permitted to hold 300 kilograms, or about 660 pounds, of low-enriched uranium under the deal. But that is not enough to produce a single weapon.
In a telephone interview, the Rosatom spokesman, Sergei Novikov, said the shipment fulfilled the requirement between Iran, the United States and five other world powers, including Russia, to remove Iran's stockpile of uranium enriched to this level.
The other fuel that can be used to make a bomb, plutonium, is made by irradiating uranium in a nuclear reactor. The process transforms some of the uranium into plutonium. The agreement requires Iran to disable its reactor at Arak and redesign it to minimize its plutonium output. The Obama administration has said these two requirements close for Iran both paths to becoming a nuclear power.
Despite the progress toward carrying out the nuclear agreement, hostility and mistrust between Iran and the United States have not abated. The Iranians have expressed anger over a new American law that curtails visa-free travel, saying it amounts to a sanction that violates the nuclear agreement, and they are threatening unspecified retaliation.
The law prohibits citizens from 38 countries, mostly in Europe, from traveling to the United States without a visa if they have visited Iran, Iraq, Syria or Sudan in the past five years, a limit that Iranians — and some Europeans — say could discourage business opportunities in Iran.
The intent of the American law is greater antiterrorism protection from the Islamic State, which has been linked to the attacks in Paris and San Bernardino, Calif. Iran opposes the Islamic State and has called Iran's inclusion in the new law nonsensical.
On Monday, Iranian news media reported that a senior parliamentary official in Tehran, Alaedin Boroujerdi, had written to counterparts in Europe, China and Russia, calling on them to object to the new visa law as a "destructive blow" to the nuclear accord. A spokesman for Iran's Foreign Ministry, Hossein Jaberi-Ansari, said Iran may take "its own steps in response."
In Washington, Mark C. Toner, a State Department spokesman, reiterated at a daily briefing that the Obama administration contends it can carry out the new visa law in a way that does not "interfere with legitimate business interests of Iran."