The "technical fault" that shut down the Bushehr plant and its 1,000-megawatt reactor on Iran's southern coast "was fixed", Mahmoud Jafari, who is also deputy head of the Atomic Energy Organization of Iran (AEOI), told ISNA news agency around midnight.

That allowed the plant to resume power generation and be reconnected, he said.

Jafari said power generation had resumed "from Sunday", and urged Iranians to "help" the Islamic republic's overburdened grid by minimising power consumption as weather forecasts predicted rising temperatures in the coming days.

The Bushehr plant was built by Russia and officially handed over in September 2013.

Russian and Iranian firms started work on two additional 1,000-megawatt reactors in 2016, with construction was expected to take 10 years.

On June 20, the AEOI had blamed "a technical fault" for the shutdown and said it had given the energy ministry one day's notice before going offline.

It said two days later that the issue was with the plant's "power generator", without explaining further.

But Iran's foreign ministry at the time described the shutdown as "routine," saying it was carried out "once or twice each year".

Bushehr going off-grid had raised concerns of worse blackouts after a string of power cuts in Iran blamed on heat, drought impacting hydro-electrical facilities, and surging electricity demand.

Iran introduced planned, rolling blackouts in May after Tehran and several other cities were hit by unannounced power cuts, sparking complaints from consumers and an apology from the energy minister.

Power cuts are not uncommon during Iran's hot summers, when air-conditioning use spikes.

Adding to the problem, the country's hydropower capacity has been hit by low rainfall.

A government report in May said precipitation was down 34 percent compared to the long-term average, and warned of reduced water supplies for the year.

Since late May, the energy ministry has regularly notified citizens of potential blackouts lasting at least two hours a day.