SEOUL - South Korea posted a sharp increase in its coronavirus cases on Wednesday as it struggled to tame its fourth wave of infections amid the spread of new virus variants strains.

The Korea Disease Control and Prevention Agency (KDCA) reported 1,725 cases for Tuesday, up by more than 500 from a day before, as more tests were conducted after the weekend.

Total infections rose to 203,926, with 2,106 deaths.

The daily tally hit a new high of 1,895 last week, partly fuelled by the more contagious Delta variant, with the fourth COVID-19 wave showing little signs of subsiding.

Health authorities were concerned that people travelled about 6.4% more last week compared with the week before, or about 34% more than in early January, in the regions beyond the capital Seoul and its neighbouring areas, largely for summer vacations.

"The movements in those regions have been increasing for three consecutive weeks," senior health official Lee Gi-il told a briefing. "There is fatigue from long periods of distancing, and it's a summer vacation season."

The government tightened social distancing curbs last week across most of the country for two weeks ahead of the peak summer holiday period.

The KDCA said on Tuesday it had detected South Korea's first two cases of the new Delta Plus COVID-19 variant, a sub-lineage of the Delta variant first identified in India.

Only a handful of countries, including Britain, Portugal and India, have reported Delta Plus cases so far. Delta Plus is still being studied, but some scientists say it may be more transmissible.

Health authorities have said several major vaccines work against the Delta variant.

South Korea will begin taking reservations for vaccines from about 2 million priority groups among people ages 18-49 late on Wednesday, including those with disabilities, gyms and private education workers, delivery people, street cleaners and call centre employees.

Some 39.3% of the country's 52 million population have received at least one shot as of Wednesday, while 14.2% have been fully vaccinated, KDCA data showed.

(Reporting by Hyonhee Shin. Editing by Gerry Doyle) ((; 822 6936 1474;))