A 2010 file photo shows Iranian security officials of the Bushehr nuclear plant in front of the nuclear power plant in Bushehr in southern Iran. EPA/ABEDIN TAHERKENAREH (Abedin Taherkenareh/ European Pressphoto Agency )

Iran shipped thousands of pounds of enriched uranium to Russia on Monday, a key commitment in the nuclear deal that had to be fulfilled before international sanctions can be lifted.

Secretary of State John F. Kerry applauded the transfer, calling it a “significant milestone” toward the deal’s implementation. He said the one shipment alone more than triples the estimated two- to three-month “breakout time” needed for Iran to acquire enough weapons-grade uranium to build one nuclear weapon.

Under the agreement reached July 14 in Vienna, Iran was required to whittle down its stockpile of low-enriched uranium to no more than 300 kilograms, about 660 pounds. Low-enriched uranium can be used to generate electrical power, but it must be enriched further to create ­weapons-grade material.

Kerry said that Iran had transferred 25,000 pounds of low-enriched uranium and materials, including scrap metal and fuel plates. State Department officials said they believe Iran has now rid itself of almost its entire stockpile of enriched uranium.

Iran has always insisted that its nuclear program is solely for peaceful purposes, despite a report by the International Atomic Energy Agency saying the country had actively worked until 2009 to design a nuclear weapon.

The nuclear negotiations aimed to ensure that Iran will have a breakout time — the time needed to amass enough weapons-grade uranium for one nuclear weapon — of at least a year. That is supposed to give enough of a lead for other countries to detect any potential cheating and decide how to respond. Monday’s shipment, Kerry said, “is an important piece of the technical equation.”

A Russian diplomat told the Tass news agency Monday that Russia had completed the procedure of withdrawing the stockpiled material, and Kerry later said the uranium had left Iran. In return, Iran will get 137 tons of natural uranium material, supplied in part by ­Kazakhstan.

The transfer suggests that Iran is well on its way to meeting its obligations under the nuclear deal it reached with six world powers, including the United States. The country still must complete a number of other steps, including dismantling centrifuges and pouring cement into the core of the Arak reactor. The International Atomic Energy Agency is responsible for verifying everything before sanctions can be eased.

Initially, Iran predicted it would finish all the work before year’s end, but U.S. experts estimated it would take many months more and probably would not happen before spring. Now it appears likely Iran will be done by the latter half of January, paving the way for sanctions to be lifted before Iran holds elections in February.

Iran’s pragmatist president, Hassan Rouhani, pursued talks that led to the nuclear deal after he was elected on a vow to bring sanctions relief. The deal has been strongly opposed by hard-liners, however, and the Iranian government is hoping to have at least the promise of imminent sanctions relief in place before election day.

Even as Iran was moving to complete one key commitment, the Foreign Ministry warned that Tehran would reciprocate if there is any breach in the deal. Last week, Iran said U.S. visa restrictions recently passed by Congress, prohibiting visa-free travel for visitors to Iran, violate the deal.

“Any steps taken outside the agreement are unacceptable to Iran, and Iran will take its own steps in response where necessary,” Foreign Ministry spokesman Hossein Jaberi Ansari told a televised news conference when asked about the U.S. law.

Mark Toner, the deputy State Department spokesman, said that the administration does not consider the new law a violation and that it will not prevent the United States from meeting its commitments on sanctions relief.