Iran shipped more than 25,000 pounds of enriched uranium to Russia on Monday, a key commitment in the nuclear deal that had to be fulfilled before international sanctions could be lifted.

Secretary of State John 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 must whittle down nine tons of low-enriched uranium it had stockpiled to 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.

Iran has always insisted 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 aim to ensure Iran will have a breakout time - the time the country would need 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 Tass news agency Monday that Russia had completed the procedure of withdrawing the stockpiled material, and Kerry later said the uranium had left Iran that very day. 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 concrete into the core of the Arak reactor.

The International Atomic Energy Agency is responsible for verifying everything before sanctions can be eased.