South Korea saw a slight decline in new coronavirus cases Monday, but cluster infections in Seoul and other regions continued to emerge, coupled with a rise in cases involving arrivals from overseas.

The 78 new COVID-19 cases, detected Sunday and down from 105 new cases a day earlier, brought the nation's total infections to 9,661 and marked the 18th consecutive day that new infections have hovered around 100 or fewer additional cases, according to the Korea Centers for Disease Control and Prevention (KCDC).

The death toll rose by six to 158, the KCDC said. One more death -- a man in his 70s from a nursing hospital in Yangju, just north of Seoul -- was reported but was not added to the official tally.

South Korea's health authorities once again stressed a "no-tolerance" policy in dealing with violators of the two-week self-isolation, adding that Korean nationals will face lawsuits and foreigners will be expelled.

Starting Wednesday, the country will enforce a two-week mandatory quarantine on all entrants from overseas in a drastic move to curb a steady rise in imported cases.

Under the new measure, all arrivals to the country regardless of nationality are required to stay in isolation for 14 days.

Visitors without local addresses are to stay in government-designated facilities at their own expense.

The KCDC said an average of around 300 people with symptoms enter the country every day, with between 20 and 30 of them testing positive for the virus.

"The rate of testing positive to COVID-19 among people with symptoms has increased recently as the so-called community spread in other countries spread," KCDC Director General Jeong Eun-kyeong said.

South Korea has been already enforcing both a two-week quarantine period and virus tests for all long-term arrivals from Europe, regardless of symptoms. Entrants from the United States have also been required to self-isolate at home for two weeks.

Foreigners who do not follow the self-quarantine rules without a valid reason will be immediately deported, health authorities said.

"Foreigners who leave their homes without notification will immediately be deported," Vice Health Minister Kim Ganglip told reporters.

A British national in his 30s violated self-isolation rules by walking around on streets without wearing a face mask even after taking a COVID-19 test, the authorities said.

The South Korean government said it is conducting an investigation to decide whether to deport him and seek compensation for possible damages.

The man had contact with a total of 23 people over the period of five days in four cities including Suwon, just south of Seoul, the KCDC said.

Of the 78 new cases reported Monday, 29 were imported cases, the KCDC said. Twenty-seven of them are South Korean nationals and two are foreigners.

The country has reported 476 COVID-19 cases coming from overseas, with 90 percent of them South Korean nationals, the KCDC said.

The government has prepared facilities that can quarantine about 2,000 foreigners at the same time for a 14-day period, Kim said.

The KCDC said fewer than 100 foreigners entering the country for short-term visits are expected to be quarantined at the facilities each day.

Daegu and North Gyeongsang Province, the nation's two worst-affected regions, reported 14 and 11 new cases, respectively, the KCDC said.

Daegu's health authorities said a total of 58 people -- including five medical staff and 53 patients -- at a hospital in the city were newly confirmed to have been infected with the virus, bringing the total at the facility to 133 and marking the single largest cluster of infections so far in the city.

Other major provinces and cities also reported infections, with 31 additional cases in Seoul and the surrounding region -- Gyeonggi Province and the western port city of Incheon.

The new infections brought the total number of cases in the metropolitan area to 947, the KCDC said.

This marked the second straight day that the number of new infections in the capital surpassed that in the southeastern city of Daegu and North Gyeongsang Province, where 7,922 cases have so far been reported.

Seoul reported 16 new cases, partly due to a church in the Guro district that has emerged as a new hot spot of infections in the capital city.

The Seoul city government said at least 28 COVID-19 patients have been linked to the church, up six from a day earlier.

In regards to the case of a family from Gimpo, west of Seoul, that retested positive for COVID-19, the KCDC said the virus was highly likely to have been reactivated.

On Saturday, a couple in their 30s and their 17-month-old child again tested positive for the virus after being discharged from a hospital.

"It is more likely for virus of the Gimpo family to have been reactivated than them being reinfected with the virus," Jeong said, also ruling out the possibility of an error in the test.

There have so far been about 10 COVID-19 patients who again tested positive after being deemed fully recovered.

The KCDC also urged people to practice social distancing through April 5 to contain the spread of the virus.

A total of 14,009 people are currently under self-isolation measures as of Sunday, which is sharply down from some 34,000 when the number peaked, the KCDC said.

Citizens are strongly urged to stay home, except for essential needs or jobs. The government has restricted religious gatherings, indoor sports activities and visits to nightclubs and other entertainment venues in a bid to stem small-scale cluster infections.

The government said it is preparing "everyday life quarantine" guidelines for people to carry out steps as part of their routines as authorities brace for a prolonged pandemic.

The guidelines are aimed at quarantining in a way that allows social and economic activities to continue.

The government also said it will form a body involving experts within this week to discuss quarantine measures that can be taken in everyday life.

Amid concerns over infection in communities, the education ministry is likely to announce a further extension of school breaks Tuesday at the earliest.

The new school year for child care centers, kindergartens and schools across the country usually begins in early March, but the date has been tentatively postponed to next Monday.

The ministry is weighing whether to adopt remote classes for all students next Monday or to start the school year for seniors and other high school students.

So far, 5,228 patients have recovered, and 4,275 are receiving treatment. A total of 372,002 people have tested negative for the virus out of 395,194 people tested since Jan. 3. The virus was first detected in the country on Jan. 20. (Yonhap)