Two Myanmar children, who arrived from abroad under the Sandbox scheme, have tested positive during their second round of testing.
Koosak Kookiattikul, chief of the Phuket public health office, said the second test conducted on day 6-7 of their stay came back positive.
He said the young visitors, aged eight and nine, were travelling with two families and all are now observing a 14-day quarantine.
The cases brought the tally of Covid-19 infected arrivals under the scheme to three since the island launched the programme to revive its tourism industry on July 1.
The first case was a man from the United Arab Emirates, who was found to be infected during testing at Phuket airport last Tuesday. On Saturday, 13 other passengers on the same flight with him returned to their country.
Dr Koosak gave assurances the disease control measures are adequate, following the two new cases, and health officials were checking their timelines over the past seven days to alert people who came into contact with the groups.
He said all visitors' movement could be traced on the Mor Chana application and at-risk people would be tested and investigated.
The island has received 3,287 overseas visitors since the reopening on July 1.
Meanwhile, local authorities tightened the screening process for domestic visitors as they reported 11 new local cases yesterday, bringing the accumulated infections to 785 since the outbreak last year.
Governor Narong Woonciew said yesterday the province had decided to impose more stringent measures after a surge in local cases.
Based on the order, all local visitors, except those aged under six travelling with parents, must meet one of the following conditions to be allowed to enter the province.
Those arriving from the 10 provinces in the maximum and strict control zone [dark red] and 24 provinces in the maximum control zone [red] must be vaccinated.
Recipients of Sinovac must have received two doses, while recipients of AstraZeneca must have had their first jab at least 14 days ago.
Those yet to be vaccinated must present a negative test result, taken no more than seven days ago, on arrival. Both the standard RT-PCR or an antigen rapid test are acceptable.
Former Covid-19 patients must have made a full recovery no less than 90 days ago.
And anyone aged under 18 and not vaccinated who enters the province for study purposes must present student ID cards issued by their schools on entry and take an RT-PCR test on arrival. Their Covid-free certificates are valid for a month.
All visitors are required to use the Mor Chana application and allow location-sharing throughout their stay. They are also advised to monitor their health and seek medical attention at the first sign of infection.
The stepped-up measures take effect from July 15-31.