Seventeen new cases of Covid-19 infection were reported on Thursday, all returnees from the Middle East.

"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 but 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 tested positive for Covid-19 in their second examination, and showed no symptoms.