Seventeen new cases of Covid-19 infection were reported on Thursday, all returnees from the Middle East and mostly asymptomatic.
"The two-digit figure is very high, but is from the daily arrival of returnees, which is expanding to 400-500," Dr Taweesilp Visanuyothin, spokesman for the Centre for Covid-19 Situation Administration, said.
He said 13 of the new cases had returned from Kuwait. Twelve were men aged 31-56, and the other a 44-year-old woman.
"All of them were found to be infected during their second test, which was about a week after their first test," he said.
"The key is quick detention, quick treatment and quick recovery."
Nine of the 13 returnees from Kuwait arrived in Thailand on May 24 and four on May 26. Five were quarantined in Bangkok and eight others in Samut Prakan province.
Infections were found in all 13 by their second Covid-19 test on Tuesday. Only one of them had symptoms, coughing and shortness of breath.
Two other new cases returned from Qatar - a 24-year-old man and woman aged 29. They arrived on May 22 and were quarantined in Samut Prakan. The woman had a cough, excessive phlegm and a headache. The man was asymptomatic.
Both patients also came up positive for the disease on their second test.
The two other new cases returned from Saudi Arabia, men aged 28 and 29. They arrived back through the Padang Besar border checkpoint in Songkhla province on May 21 and 25 and were quarantined in the southern province. They tested positive for Covid-19 in their second examination, and showed no symptoms.
So far 174 Thais had returned from Kuwait and 18.29% of them tested positive for Covid-19. Seventy-nine returned from Saudi Arabia and the infection rate was 7%. From Qatar, 216 arrived and 5.49% of them were infected, Dr Taweesilp said.
"Thais were infected in other countries but they have the right to return home," he said. Over the past month, most returnees were workers and students.
The total number of Covid-19 cases had risen to 3,101, 96% of whom had recovered. There were no new recoveries in the previous 24 hours. The death toll remained unchanged at 58.
Global Covid-19 cases stood at 6.57 million, up by 82,939 over the previous 24 hours, with 387,957 deaths, 5.9% of the total and up by 5,545 in 24 hours.
The United States had the most cases at 1.90 million and the most deaths at 109,142.