South Korea ordered government offices to turn off their air-conditioning as two power plants stopped operations Monday, a day after a minister warned of an imminent national energy crisis.

The Dangjin III plant, with a capacity of 500,000 kilowatts, was taken offline by mechanical issues and will likely remain shut for a week, a spokesman for the state power distributor Korea Power Exchange (KPE) said.

Technical problems also shut down the nearby Seocheon power plant on Monday morning. Although operations resumed after an hour, the plant is only working at half its 200,000-kilowatt capacity, the spokesman said.

The timing could hardly be worse, with South Korea in the grip of an extended heatwave and a lengthy disruption in its nuclear power sector.

"We are facing potentially our worst power crisis," Trade, Industry and Energy Minister Yoon Sang-Jick said Sunday.

"We may have to carry out a rolling blackout... if one single power plant goes out of operation," Yoon said, appealing to factories, households and shops to curb consumption over the next three days.

The last time the government was forced to resort to nationwide load shedding was in September 2011, when unexpectedly high demand pushed power reserves to their lowest level in decades.

If national reserves drop below 2.0 million kilowatts, it triggers an automatic alert requiring all government offices to turn off air conditioners, lights and any non-essential devices.

In a pre-emptive move Monday, the energy ministry ordered such measures effective immediately, even though the key reserve mark had not been breached.

Describing the current situation as "extremely urgent", the ministry also ordered government offices to turn off water coolers and staff to use staircases where possible, rather than elevators.

The ministry added it would tighten monitoring on shopping malls, which face fines for bringing indoor temperatures below 26 degrees Celsius.

Higher than normal summer temperatures -- forecast to last at least another week -- have resulted in a sustained energy consumption spike.

At the same time, South Korea's nuclear industry is struggling to emerge from a mini crisis which has forced the shutdown of numerous reactors -- either for repair or as the result of a scandal over forged safety certificates.

The country has 23 reactors which are meant to meet more than 30 percent of electricity needs. Currently six reactors are out of operation.