South Korea will begin treating coronavirus patients with Pfizer's antiviral pills, health officials say, as concern mounts over the spread of the highly contagious Omicron variant.

At least 21,000 of the pills, called Paxlovid, arrived on Thursday to be distributed to some 280 pharmacies and 90 residential treatment centres, the Korea Disease Control and Prevention Agency (KDCA) said.

"In light of Omicron's much higher infectiousness, the medication should play a meaningful role in restraining the number of patients who would develop critical symptoms even if the strain is relatively less severe," Kim Ki-nam, a KDCA official, told a briefing.

The medication will be used to treat more than 1000 people a day, with priority groups including patients at high risk of severe illness, those aged 65 or older and those with reduced immunity, the KDCA said.

Another 10,000 of the pills are expected to arrive later in the month.

Paxlovid was nearly 90 per cent effective in preventing hospitalisations and death rates, and data suggested it retains its effectiveness against Omicron, Pfizer has said.

South Korea has been exploring additional pharmaceutical tools to head off a surge of infections caused by the Omicron variant. It approved the use of Novavax Inc's vaccine on Wednesday.

Since the pandemic began, the country of 52 million people has been among the coronavirus mitigation success stories, with 679,030 cases in all and 6210 deaths, achieved largely with masks and social distancing.

The KDCA reported another record for imported COVID-19 cases for Wednesday, at 391, among a tally for the day of 4167 infections.

The number of South Koreans who tested positive after attending a consumer electronics trade show in Las Vegas last week rose to 119, including some Omicron cases, a KDCA official said.

The Omicron variant makes up a small fraction of its overall cases but the share has more than tripled to about 12.5 per cent over the past two weeks.

Health officials have warned it could this month become dominant, as it has in the United States and much of Europe, and daily tallies could soar up to an unprecedented 20,000 next month.

The government will decide on Friday whether to extend social distancing rules, which were reinstated in mid-December after daily infections hit new highs of nearly 8000.

Nearly 90 per cent of South Korean adults have been fully vaccinated and 55 per cent have had a booster shot as of Wednesday, KDCA data showed.