South Korea's daily new virus cases stayed in the 700s for the second day Sunday, while authorities tighten their guard against a virus resurgence in the greater Seoul area.

The country reported 743 more COVID-19 cases, including 662 local infections, raising the total caseload to 160,085, the Korea Disease Control and Prevention Agency (KDCA) said.

The latest figure is down from 794 on Saturday and 826 on Friday, a nearly six-month high due to the sharp spike in new cases in the Seoul metropolitan area.

However, it marked this year's highest level for any Sunday, when new cases usually remain lower than weekdays due to fewer tests.

The country added one more death, raising the death toll to 2,026.

New cases spiked to close to levels unseen since January, when the nation was gripped by the third wave of the pandemic, with many cases from schools, private institutions, restaurants, indoor gyms and entertainment venues.

New cases in the greater Seoul area, home to half of the nation's 51 million population, have accounted for over 80 percent of the total over the past week, and nearly half of them were in their 20s and 30s, the KDCA said.

Health authorities said the nation's inoculation drive has sharply flattened the virus curve among the elderly population, but the proportion of younger patients has been on the rise as they are not yet eligible for vaccinations.

While those aged over 60 and essential workforce have been mostly receiving jabs, high school seniors and faculty members, as well as those aged between 55 and 59, will be eligible for vaccinations later this month.

Authorities express concerns over a further spike after the nation's major umbrella labor union pressed ahead with a massive street rally Saturday in central Seoul despite the government's warning of a stern response.

The Korean Confederation of Trade Unions (KCTU) said around 8,000 members participated in the rally.

Prime Minister Kim Boo-kyum expressed regret over the KCTU's rally despite the government's repeated requests, urging attendees to receive tests if they show symptoms and warning of strong actions for those who violated public health regulations.

"The spread of COVID-19 is not slowing down, and the situation in the Seoul metropolitan area is particularly serious," Kim said in a virus response meeting held at the Seoul government complex.

"Authorities will closely monitor the latest situation in the wider Seoul area and will discuss additional antivirus measures."

The government has delayed adoption of eased distancing measures by one week till July 7 amid the recent virus resurgence, and authorities said they will closely monitor the latest situation before deciding whether to implement the relaxed virus curbs in the wider Seoul area.

Under the new rules, the ban on private gatherings of five or more people and the 10 p.m. curfew on restaurants had been expected to be lifted.

A total of 15.35 million people, or 29.9 percent of the country's population, have received their first shots of COVID-19 vaccines. The KDCA said 5.3 million people have been fully vaccinated, taking up 10.4 percent of the population.

The country currently administers two-dose vaccines from AstraZeneca, Pfizer and Moderna, as well as Janssen's single-shot vaccine.

Of the newly confirmed domestic cases, 286 were from Seoul, 227 from the surrounding Gyeonggi Province and 28 from the western port city of Incheon.

A total of 59 have tested positive in connection with a restaurant in western Seoul, mostly foreign English teachers.

Eight private English institutes in the Seoul metropolitan area have reported a combined 242 patients, who are believed to have had contact with the infected teachers, authorities said.

There were 81 additional imported cases, with 75 of them from Asian nations excluding China.

The number of imported cases was at a nearly one-year high, raising the alarm among authorities over the spread of variant cases in the nation.

Among them, 39 were from Indonesia, which reported a sharp surge in the more contagious delta variant first identified in India.

Currently, people from India and Indonesia must undergo a two-week quarantine period upon their arrival here even if they have completed vaccinations and tested negative, as part of quarantine measures against the different type of variant rampant in the two Asian nations.

A total of 263 patients have been infected with the delta variant in the nation, and 94 others have come in close contact with them, which could potentially raise the variant cases, according to the KDCA.

The number of patients in serious or critical condition came to 143, down one from the previous day.

The total number of people released from quarantine after making full recoveries was 149,614, up 482 from a day earlier. (Yonhap)