Nuclear technicians have finished removing the core of the Iran's only nuclear heavy water reactor as part of Tehran's obligations under its nuclear deal with world powers, Iranian state television reported Thursday.

The removal of the core of the nearly completed Arak reactor is a key step before crippling international sanctions on Iran are lifted. The work must still be verified by outside experts.

Under the deal reached last summer, the heavy-water reactor is to be re-engineered so that it produces only minute amounts of plutonium, like enriched uranium a potential pathway to nuclear arms. That involves exchanging the core and other major modifications.

The spokesman for Iran's atomic department, Behrouz Kamalvandi, announced the completion of the work on the Arak reactor on Thursday.

"About an hour age, our job was finished," he said. State TV reported that the holes left after the core was removed have been filled with concrete.

International inspectors will now verify the job and will send their report to the International Atomic Energy Agency, Kamalvandi said.

The core itself would be kept as a symbol of Iran's nuclear achievements, he added.

Under the deal, reached last July, Iran was required to ship out most of its stockpile of enriched uranium, a material that can be used to make bombs, and take apart thousands of the centrifuges that enrich the material. It also had to redesign Arak, rendering it incapable from producing weapons-grade plutonium under normal operation. Most of the nuclear restrictions last 10 or 15 years.

While Iran accomplished all of the biggest tasks in recent weeks, several technical issues remained. Officials had spoken of lingering questions related to Iran's uranium and plutonium programs, including over the properties of certain centrifuges Iran would be permitted to maintain for research purposes.

Iran has insisted it needs the Arak heavy water reactor for production of medical isotopes. It denies it had any intention to build nuclear weapons.

Iran's Fars news agency, which is close to the country's hard-liners, on Monday reported that technicians had already dismantled the core of the Arak reactor and filled it with concrete.

Deputy nuclear chief Ali Asghar Zarean denied that report a day later, saying that Iran "will not apply any physical change in this field until a final agreement is finalized."

Iranian hard-liners are opposed to the nuclear deal. They argue that the so-called "disabling" of Arak is a slap Iran's face and see it as evidence that moderate President Hassan Rouhani has made too many concessions to the West.

On Wednesday, U.S. officials said Iran could comply with last summer's nuclear deal as early as Friday, requiring the United States and other nations to immediately suspend billions of dollars' worth of economic sanctions on the Islamic Republic.

In Vienna, a senior diplomat from one of the six countries that cut the deal with Iran said Wednesday that it would be formally declared implemented -- most probably on Friday. The official and diplomat spoke on condition of anonymity because they were not authorized to discuss the matter publicly.

Zarean, the Iranian deputy nuclear chief, also said Tuesday that once modifications at the plant are done are done and Arak goes online, Iran hopes to export excess heavy water produced there to the U.S. through a third country, for uses in research. He added that Savannah River National Laboratory near Jackson, South Carolina, has recently certified high purity of heavy water produced by Iran.

Iran is still expected to produce some 20 metric tons (22 tons) of heavy water at Arak a year. It has said it would domestically consume about 6 tons for medical isotopes and is looking to export the rest.