South Korean President Moon Jae-in has proposed a joint project with China to use artificial rain to clean the air in his country, where an acute increase in pollution has caused alarm.

Moon also instructed government officials on Wednesday to speed up the retirement of old coal-burning power plants, according to his spokesman, Kim Eui-kyeom.

Seoul has been struggling to tackle the rise in air pollution that experts have linked to China's massive industrial activity and emissions from South Korean cars. Fine dust levels in South Korea have hit new highs over the past week, prompting people to wear masks while commuting under thick-gray skies that online users have compared to scenes from the movie "Wall-E."

When asked about Moon's proposal, Chinese foreign ministry spokesman Lu Kang said cooperation would be good but downplayed Seoul's claim that China is a major source of its pollution.

"I wonder if the South Korean side has any basis that its smog is from China," Mr Lu said, noting that fine dust readings have been higher in Seoul than in Beijing recently. "All countries realize that the cause is very complicated."