SEOUL (BLOOMBERG) - South Korea reversed a decision to relax mask rules in the capital Seoul and its metropolitan area after a spike in Covid-19 cases, driven by people in their 20s and 30s and the rapidly spreading Delta variant.

Masks will be compulsory for everyone, including those who have been vaccinated, in the city and its surrounding areas because of the recent rise in cases, the Korea Disease Control and Prevention Agency said in a statement on Sunday (July 4).

The announcement comes after the government said in May that it would lift its mask mandate for people outdoors if they have been inoculated.

About 15.3 million people in South Korea have received the first dose of the vaccine.

Covid-19 is spreading at a "menacing" speed in the Seoul area, with the Delta variant increasing rapidly, Health Ministry official Sohn Young-rae said at a briefing.

Outdoor drinking past 10pm will also be banned in the Seoul area, the agency said.

The city's riverside parks have been bustling with young people socialising and drinking after restaurants, bars and pubs close at 10pm, as part of social distancing restrictions.

The latest spike in infections has been fuelled by people in their 20s and 30s living in Seoul or the neighbouring Gyeonggi province, who account for 81 per cent of Covid-19 patients, according to the government agency.

"We are seeing eroding caution against the pandemic societywide," Ms Sohn said. "The speed of the Delta variant's spread is also accelerating."

In the past week, the country's Covid-19 cases have risen to a daily average of 531.3 people, 46 per cent higher than the number a week earlier.

The authorities reported 743 new infections in the latest 24-hour period on Sunday.