South Korea said Friday minor cracks were found at a nuclear reactor that have not caused a leak but still mean the facility will be kept offline, prompting fears of power shortages in the coming winter.

The cracks were found during maintenance work on control rod tubes at a unit in the Yeonggwang nuclear complex, where two other reactors were recently shut down for several months to replace unvetted components.

Half of Yeonggwang's six reactors will now be offline until January at the earliest, the country's nuclear watchdog said, raising concerns of a pinch in power supply during the harsh winter months.

South Korea's 23 nuclear reactors generate around 35 percent of the country's electricity.

The cracks were found in a reactor that had been shut down on October 18 for regular maintenance work, which normally takes just over a month.

"This is the the first time these type of cracks have been discovered, but there was no radioactive leak," Nuclear Safety and Security Commission (NSSC) spokeswoman Shim Eun-Jung told AFP.

"We believe the extent of the cracks is not serious, although a further inspection is required," Shim said, adding that the reactor would remain offline until early January at the earliest.

On Monday, the government said it was shutting down two other reactors at Yeonggwang to replace thousands of "non-core" parts that had been provided with forged quality and safety warranties.

Both are expected to remain offline until early January, and the NSSC said Wednesday that components at all the country's 23 reactors would have to be inspected.

The government has vowed to stick to its nuclear power programme despite public concerns arising from last year's nuclear disaster in Japan. South Korea plans to build an additional 16 reactors by 2030.