TEHRAN, Iran (AP) — Iran's deputy nuclear chief on Tuesday denied a report that the core of the country's nearly finished heavy water reactor has been dismantled and filled it with concrete as part of Tehran's obligations under the nuclear deal with the West.

Ali Asghar Zarean, in remarks to state TV said that Iran will first sign an agreement with China to modify the Arak reactor, a deal that is expected next week.

"Definitely, we will not apply any physical change in this field until a final agreement is finalized," Zarean added, without specifically mentioning the Fars news agency report.

On Monday, Fars said that technicians had dismantled the core of the Arak reactor and filled it with concrete. The agency, which is close to Iranian hard-liners, cited unnamed sources for the report.

Under the landmark nuclear deal that commits Tehran to significant limits on its nuclear activities for over a decade in exchange for relief from crippling economic sanctions, Iran must redesign the Arak reactor so it can't produce plutonium for nuclear weapons — though it will still produce small amounts of plutonium and heavy water.

Iran has insisted it needs the heavy water reactor for production of medical isotopes.

Hard-liners in Iran, who oppose Iranian President Hassan Rouhani and the nuclear deal with world powers, argue that the so-called "disabling" of Arak is a slap in the face of Iran and allegedly evidence of Rouhani having given too many concessions to the West in return to little.

It's not clear what the modification process at Arak will involve, but officials in the past have said that some parts of the reactor need to be filled with cements because of safety concerns.

Zarean also said that once modifications 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.