SEOUL--South Korea broke its daily record for coronavirus infections for a second straight day on Thursday with more than 5,200 new cases, as pressure mounted on a health care system grappling with rising hospitalizations and deaths.

The rapid delta-driven spread comes amid the emergence of the new omicron variant, which is seen as potentially more contagious than previous strains of the virus, and has fueled concerns about prolonged pandemic suffering.

South Korea confirmed its first five omicron cases Wednesday night linked to arrivals from Nigeria, prompting the government to tighten its border controls.

The country will require all passengers arriving from abroad over the next two weeks to quarantine for at least 10 days, regardless of their nationality or vaccination status. South Korea since Sunday had already banned short-term foreign travelers arriving from eight southern African nations, including South Africa, and has now extended the same rules to foreigners coming from Nigeria.

Jung Eun-kyeong, commissioner of the Korea Disease Control and Prevention Agency, said the government plans to conduct omicron testing on all international passengers who test positive for the coronavirus and will work with biotech companies to develop new tests that could detect the variant faster.

Anyone who comes in close contact with a person infected with omicron will be required to quarantine for a minimum of two weeks, even if they are fully vaccinated, she said in a briefing.

While the alarm over omicron has forced governments around the world to tighten border controls, scientists say it is not yet clear whether the variant is more contagious or dangerous than previous strains of the virus, including delta, which has devastated South Korea in recent weeks.

The KDCA said more than 4,100 of the 5,266 new coronavirus cases came from the capital, Seoul, and its nearby metropolitan region, where officials say 90% of intensive care units designated for COVID-19 are already occupied. A record 733 virus patients are in serious or critical condition, while the country’s death toll is now 3,705 after 47 patients died in the past 24 hours.

Health experts have called for the government to reimpose strict social distancing rules that were eased in November to improve the economy, raising concerns that hospital systems could become overwhelmed.

While the government previously resisted calls to reinforce strict virus restrictions, citing public fatigue and the impact on livelihoods, there is expectation that officials could announce stronger social distancing rules as early as Friday, which could include limits on private social gatherings and indoor dining hours.

Jung pleaded for people to cancel all unnecessary social gatherings and for senior citizens to get booster shots as early as possible. While less than 8% of South Koreans in their 60s or older aren’t fully vaccinated or vaccinated at all, these people have accounted for more than 42% of virus patients in serious or critical condition, she said.

Despite public attention migrating toward omicron, Jung stressed that delta is still the biggest menace at hand. She worried that people would delay their booster shots while waiting for pharmaceutical companies to develop new vaccines targeting omicron.

“It’s not yet clear how exactly omicron would affect the effectiveness of vaccines, but it’s not like existing vaccines would be completely useless or provide significantly less protection” against the new variant, Jung said. “A quick rise in immunity provided by a third shot would be beneficial in responding to the omicron variant.”

The KDCA said the country’s first five omicron cases were a couple who arrived from Nigeria on Nov. 24, a man who drove them home, and two women who also traveled to Nigeria and returned home on Nov. 23. Health workers are conducting genetic sequencing tests on four other people who are suspected of omicron infection, including one of the couple’s children and relatives of the friend who drove them home.

Health workers are also monitoring hundreds of people who arrived on the same flights with the omicron carriers or had close contact with them. Among the nine who were confirmed or suspected of omicron infection, only the couple who arrived on Nov. 24 was vaccinated, the KDCA said.