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

At least 21,000 of the pills, called Paxlovid, will arrive on Thursday and be sent out to some 280 pharmacies and 90 residential treatment centres, Reuters quotes Korea Disease Control and Prevention Agency (KDCA) as saying.

The medication will be used to treat more than 1,000 people a day, with priority groups including patients with a high chance of developing critical symptoms, those aged 65 or older and those with reduced immunity, the KDCA said.