BANGKOK (REUTERS) - Thailand reported over 500 cases of coronavirus from the centre of its seafood industry where there are many migrant workers on Saturday (Dec 19), in by far the biggest one-day rise in a country that had previously brought the epidemic largely under control.

The new cases were reported in Samut Sakhon province, south-west of Bangkok, where four infections were reported at a shrimp market on Friday - the first a 67-year-old woman.

Disease Control Department director-general Opas Karnkawinpong told a news conference that 516 new cases were found after testing that included 1,192 migrant workers.

More than 90 per cent were asymptomatic, he said.

A total of 535 new cases reported on Saturday, bringing Thailand's total number to more than 4,800 with 60 deaths.

The majority of the migrant workers in Samut Sakhon are from Myanmar, which has suffered a far worse outbreak of coronavirus than Thailand, where health authorities credit early action with limiting the spread of the virus.

"While there is a likelihood of finding more infections in crowded foreign communities around the shrimp market, they are low-risk groups because they are working age and healthy," Dr Opas said.

The province was ordered closed until Jan 3.

Local cases have previously largely been found in people observing quarantine after having been in close contact with an infected person.

Most of Thailand's recent cases have been imported.

The surge in cases comes just as Thailand is seeking to revive a tourist industry that has been devastated by the epidemic.

On Thursday, it eased restrictions to allow more foreign tourists to return.

The government predicts about 8 million foreign tourists in 2021 after 6.7 million expected this year. Last year's foreign visitors were a record of nearly 40 million.