People wearing thick coats and mufflers cross the street at Gwanghwamun in central Seoul, Thursday. Yonhap

A severe cold wave gripped Korea on Thursday, with temperatures plunging to this season's lowest levels in many regions, according to the weather agency.

The cold wave is forecast to become even more severe on Friday, with temperatures expected to decrease even further to as low as minus 12 degrees Celsius in Seoul, minus 9 degrees in Busan and minus 11 degrees in Incheon.

The Korea Meteorological Administration said temperatures in Seoul plummeted to minus 10.2 degrees early Thursday morning amid a cold wave advisory that took effect on Wednesday night.

An advisory is issued when a morning low of minus 12 degrees or less is expected for two or more consecutive days.

The lowest temperature was minus 10.3 degrees in Incheon, minus 9.3 degrees in Daejeon, minus 6.9 degrees in Gwangju, minus 6.9 degrees in Ulsan and minus 6.9 degrees in Busan. It dropped to as low as minus 16.9 degrees in Daegwallyeong, a highland county in Gangwon Province, and even below minus 20 degrees in some mountainous regions.

"Many parts of the country recorded their lowest temperatures of this winter," the weather agency said.

A couple takes a selfie near an icy hill at a park in Daegu, Thursday. Yonhap

"On Saturday, early morning temperatures in Seoul are expected to fall to about minus 8 degrees. The next day, the day's low would be about the same level as recorded last year," Kim Young-joon, an analyst at the weather agency, told reporters. "The cold spell is expected to continue through Saturday countrywide … It is advisable to stay indoors and pay attention to health during this cold wave. It is also advisable to keep water pipes from freezing and bursting and to be careful on icy roads."

According to data from the Korea Disease Control and Prevention Agency, 138 people were taken to 500 emergency rooms nationwide due to cold-related illnesses between Dec. 1 and Jan. 6. Four of them died, with cold-related reasons suspected as the cause of death. Health authorities worry about a possible jump in cases as temperatures continue to fall.

An airplane is parked at Jeju International Airport, Thursday. At least 27 flights were canceled nationwide due to snow. Yonhap

In the Korean Peninsula's southern regions, including Jeju Island, dozens of flights and ships were canceled or postponed due to snow.

According to the national disaster management headquarters under the Ministry of the Interior and Safety, at least 136 flights were canceled that day. Meanwhile, 77 passenger ships, including ones servicing Incheon Port and Baengnyeong Island, canceled operations.

Amid rising worries about safety accidents associated with the cold weather and snow, 197 walking trails at eight national parks were temporarily closed, officials said.

No major incidents related to snow have been reported so far. But as heavy snowfall is forecast through Saturday in some southern regions including Jeju, officials said they will continue to monitor the situation closely.