Ebrahim Raisi speaks after casting his ballot at a polling station in Tehran, Iran, June 18, 2021. Photo: Xinhua

Iran's only nuclear power plant has been brought back online, its manager said early Monday, after two weeks off-grid amid a power shortage and rolling blackouts across the Islamic republic.

The Bushehr plant's shutdown was initially blamed on a "technical fault" that required repairs followed by conflicting reports that it was a regular maintenance operation.

The plant went offline as Tehran and world powers in Vienna held talks in an attempt to revive a hobbled 2015 agreement on Iran's nuclear program that was torpedoed by the US.

It returns to the grid as major cities across Iran including the capital Tehran are experiencing frequent blackouts blamed on high summertime demand exceeding production levels.

The "technical fault" that shut down the Bushehr plant "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 be reconnected to the national power grid and resume production.

Jafari said power generation had resumed Sunday, and urged Iranians to "help" the overburdened grid by minimising power consumption as high temperatures are forecast in the coming days. The plant on Iran's southern coast and its 1,000-megawatt reactor were built by Russia and officially handed over in September 2013 after years of delay.

Russian and Iranian firms started work on two additional 1,000-megawatt reactors in 2016, with construction 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.

Bushehr plant chief Jafari said in late March that Iran was having a difficult time obtaining supplies to run Bushehr because of US sanctions, and warned of an imminent shutdown "if no solution is found."

A government report in May said precipitation was down 34 percent compared to the long-term average, and warned of reduced water supplies for 2021.