Eight Thais, from two tour groups, tested positive for the Omicron coronavirus variant after returning from an annual pilgrimage to the Islamic holy city of Mecca in Saudi Arabia.
Authorities have urged people not to panic over the imported cases as those infected have been quarantined.
Preeda Chueaphudee, adviser to the Chularatchamontri -- the country's Muslim spiritual leader -- said about 30 Thais returning from a pilgrimage arrived at Suvarnabhumi airport on Dec 15.
They were quarantined at a hospitel (hotel-cum-hospital) on Soi Ramkhamhaeng 5 in Bangkok. Doctors conducted the tests, he said, adding that results showed seven were infected with the Omicron variant. Mr Preeda said there are also six people who are deemed to be at high risk as they came in close contact with the seven.
All are now quarantined at the hospitel which has a contract with a hospital, Mr Preeda said.
After leaving Saudi Arabia, the returnees flew to Qatar where they transited before arriving at Suvarnabhumi airport, he said.
Some had already developed symptoms such as fever while in Saudi Arabia. The returnees come from various provinces, such as Nonthaburi and Ayutthaya, he said.
Kusak Kukiatkul, the Phuket public health office's doctor, said another group of 137 Thai returnees from a pilgrimage to Mecca arrived in Phuket and underwent testing for Covid-19 on Dec 13.
Five tested positive for Covid-19 and were quarantined for five days in Phuket.
After the quarantine, the five, who were categorised as green patients with mild or no symptoms, were allowed to seek further treatment at hospitals in other southern provinces under close surveillance by health authorities, Dr Kusak said.
However, Phuket's centre for medical sciences reported on Dec 17 that one of the five returnees tested positive for Omicron. The patient is now quarantined and being treated at Khok Pho Hospital in Pattani, Dr Kusak said.
He added that four foreign visitors to Phuket also tested positive for Omicron on Dec 17. They arrived on four separate airlines and entered Phuket under the Test & Go scheme. They are an American, a Swede, a Tunisian, and a German.
All are now quarantined and being treated in a hospital in Phuket, Dr Kusak said, adding that about 20 at-risk people who came into contact with the four are also being quarantined.
Anurak Saraphap, Pattani's public health doctor, said the 137 Thai returnees underwent RT-PCR testing in Saudi Arabia before flying to Phuket where they underwent further RT-PCR testing.
He also urged residents in Pattani not to panic over the imported case of the Omicron variant but to continue taking precautions against the virus.
The Pattani public health official also said the Omicron patient who is now quarantined at Khok Pho Hospital is an imported case, not a local transmission case.
Local sources said people in Pattani were also concerned about the possible spread of the Omicron variant from Thai returnees from Malaysia.
Currently, some 57 Thai returnees from Malaysia are still in quarantine, but no Omicron infections have been detected among them.
However, the cases have prompted provincial authorities to step up screening of arrivals from foreign countries to keep the Omicron variant at bay, the sources said.
Meanwhile, Thailand logged 2,899 new Covid-19 cases and 22 more fatalities during the previous 24 hours, the Public Health Ministry announced on Sunday.