Yellow and fine dust blanketed South Korea on Friday with a thick haze of pollution categorized as "very bad" in the central region of the country, prompting the state weather agency to issue advisories.

Yellow dust advisories for the greater Seoul area, Gangwon and South Chungcheong provinces were raised to "caution," the second-lowest level in the government's four-stage crisis alert as of 8 a.m. Friday, according to the Ministry of Environment.

A yellow dust warning is issued when the average hourly concentration of fine particulate matter or PM10 stays above 300 micrograms for longer than two hours.

The Environment Ministry said it has issued North Gyeongsang Province the "attention" level against yellow dust, the lowest in its warning system.

The "attention" level is issued when particulate matter concentrations are forecast to be "very bad." 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."

As yellow dust originating near the Inner Mongolia region is being carried into the country by northwesterly winds, the fine dust level across the country is expected to remain at "bad" to "very bad" throughout the day.

The yellow dust is expected to continue to cover the country as it moves southeastward with the northwesterly winds and is expected to continue to affect the country through Saturday, according to the weather authorities.

Korea's national weather agency advised people with respiratory or cardiovascular illnesses, the elderly and children to stay indoors. People should use masks when going outside when fine dust levels are high. The agency also advised avoiding rain, as it contains fine dust particles.