South Korea's state-run electricity distributor issued a power shortage alert Tuesday due to a surge in electricity consumption caused by high temperatures, the second such move in as many days.

The Korea Power Exchange issued an initial warning at 11:20 a.m. after the country's electricity reserve dropped to below 4 million kilowatts.

The company later upgraded the warning to its third-highest level of "attention," which is triggered when the reserve level drops to between 2 million and 3 million kilowatts. The second-highest warning of "caution" is issued when the reserve falls to less than 2 million kilowatts.

If the electricity reserve runs out, it may cause a nationwide blackout, which could take weeks or months to repair, according to the Ministry of Knowledge Economy that handles the country's energy affairs.

A temporary blackout that affected about 2 million households in September 2011 caused billions of won in damages and a 61-billion-won damage suit against the government despite its small scale compared to a nationwide blackout.

While trying to employ all available sources of power, the government on Monday restarted an aged nuclear reactor that had been shut down since March for a safety inspection.

The reactor was declared safe by a state commission early last month, but the resumption of its operation had been prevented by public concerns over the safety of the reactor, whose initial 30-year lifespan ran out but was extended by 10 years in 2008.

The latest surge in energy consumption is largely caused by an unprecedented heat wave, coupled with high humidity levels that are forcing more people to turn to their only line of defense -- air conditioners.

Seoul has been experiencing its longest streak of so-called tropical nights, where nighttime lows remain over 25 degrees Celsius, since July 27. (Yonhap News)