A yellow dust storm that originated in the Gobi Desert in northern China and Mongolia on Friday blanketed most of South Korea, including the greater Seoul area, the state weather agency said.

A fine dust advisory, issued for Seoul at 1 p.m., was upgraded to a fine dust warning one hour later, as the average hourly concentration of fine dust particles smaller than 10 micrometers in diameter, known as PM 10, surged to 610 micrograms per cubic meter in the capital Friday afternoon.

A fine dust warning is issued when the PM 10 figure stays above 300 micrograms for longer than two hours.

A fine dust advisory was replaced by a fine dust warning in Gyeonggi Province that surrounds the capital, Incheon and South Chungcheong Province as the yellow dust storm moved southeastward in the early afternoon, according to the state-run Air Quality Forecasting Center. As of Friday evening, either a fine dust warning or advisory was issued in parts of North Jeolla, Gangwon, North Chungcheon Province.

The center said the latest yellow storm is expected to spread to almost all other parts of South Korea and affect the entire nation until Sunday.

As of 7 p.m, the average hourly PM 10 density soared to 555 in Seoul and 758 in Suwon, south of Seoul.

The figure rose to 520 in northern Chuncheon, Gangwon Province, 713 in western Cheongju, North Chungcheong Province and 686 in Gunsan, North Jeolla Province as of 7 p.m.

The weather agency also issued yellow storm warnings over the five islands in the West Sea near the border with North Korea, including Yeonpyeong Island, where the hourly PM 10 density reached 817 as of 7 p.m, as well as part of South Chungcheong Province.

It is the country's first issuance of a yellow dust storm warning for its inland regions in 13 years.

On Saturday, the PM 10 density will remain at the "very bad" level in the capital, Chungcheong and Jeolla provinces and the southern resort island of Jeju, the center said.

The weather authorities here categorize concentrations of PM 10 between zero and 30 micrograms as "good," between 31 and 80 as "normal," between 81 and 150 as "bad" and more than 151 as "very bad."

In a related move, the Ministry of Environment said the third-highest level of "caution" in the government's four-stage crisis alert against yellow dust was enforced for Seoul, Incheon, Gyeonggi Province and South Chungcheong Province as of 2 p.m.

Six other regions -- Gangwon Province, Daejeon, Sejong, North Chungcheong Province, Gwangju and North Jeolla Province -- were placed under the lowest "attention" level, the ministry said.

The yellow dust "caution and "attention" levels are enforced when the daily PM 10 average is expected to stay above 300 and 150 micrograms, respectively, for long than two hours.

The ministry asked that all outdoor workers wear masks and weak and vulnerable people be protected from air pollution. (Yonhap)