South Korea will expand tougher Covid-19 restrictions on private gatherings to outside the Seoul metropolitan area, as the country struggles to contain its worst outbreak, its prime minister has said.

"It's been a week since the toughest level of distancing curbs have been imposed in the metropolitan area but the number of confirmed [virus] cases is rarely decreasing ... pre-emptive measures are needed for now," Prime Minister Kim Boo-kyum told a meeting with health officials, according to Reuters.

From Monday until August 1, current rules in Seoul limiting private gatherings to four people will apply to the rest of the country.