"Today, the process of pouring concrete for the foundation of the reactor started at the Isfahan site," said Mohammad Eslami, head of Atomic Energy Organisation of Iran, according to state media IRNA.

The Isfahan nuclear research centre in central Iran is already home to three reactors. The new 10-megawatt research reactor is being constructed to create a powerful neutron source, IRNA said.

It would have a variety of applications, including fuel and nuclear material tests and the production of industrial radioisotopes and radiopharmaceuticals, it said.

Tehran has been under biting US sanctions since 2018, when then US president Donald Trump withdrew his country from a landmark nuclear deal which granted Iran sanctions relief in return for curbs on its nuclear activities designed to prevent it from developing an atomic warhead.

Iran has always denied any ambition to develop a nuclear weapons capability, insisting that its activities are entirely peaceful.

In January, the director general of the UN's IAEA nuclear watchdog, Rafael Grossi, lamented that Iran was "restricting" cooperation with the agency and called the Iran nuclear situation "frustrating".

On Thursday, Eslami announced the construction of a nuclear power plant complex in Sirik, on the Strait of Hormuz, comprising four individual plants with a combined production capacity of 5,000 megawatts.

"We must reach the production capacity of 20,000 megawatts of nuclear power in the country" by the year 2041, Eslami said while on a trip to the region with Iranian President Ebrahim Raisi.

Only five countries -- the United States, France, China, Russia and South Korea -- currently have more than 20,000 megawatts of nuclear capacity installed.

The Sirik plants are expected to be fully operational by 2031, IRNA reported. Iran currently has one operational nuclear power plant, in Bushehr, capable of producing 3,000 megawatts.