Laos and Myanmar -- two of South-East Asia's least developed countries -- both claim they have no cases of COVID-19, which has killed more than 15,000 people globally.
The number of confirmed infections in Thailand, which employs millions of migrant workers from its neighbours, more than doubled over the weekend to a regional high of 721.
With virtually all public spaces shuttered in Bangkok, many workers have been left jobless.
Although the government has urged the public not to travel home, thousands of Burmese workers waited on Monday at a northern border gate to cross the frontier.
After entering Myawaddy town -- which borders Thailand's Tak province -- workers walked past temperature scanners and were questioned by nurses about their health.
More than 9,000 were expected to have crossed into Myanmar by the end of Monday after the surge began over the weekend.
Tak provincial governor Unsit Sampuntharat said the Thai-Myanmar "friendship" bridge would then be closed.
Bangkok's three main bus stations also saw more than 84,000 travellers over the weekend, said Jirasak Yaowasakul, managing director of government-owned Bangkok Bus company.
"They are mostly travelling to the north and northeast provinces bordering Laos," he said, adding his company -- unlike many private businesses -- is spacing out passengers to reduce transmission risks.
A Burmese woman employed at a Thai fish canning factory told AFP she returned at the behest of her parents.
"They were worried about me and they wanted me to come back," Khine Khine Mar said as she waited at Myawaddy's checkpoint.
People from Laos, Myanmar and Cambodia make up the bulk of the approximately four million migrant workers in Thailand.
The number of known virus cases in Cambodia jumped to 84 on Sunday after a group of 29 French tourists tested positive.
There is increasing scepticism in under-developed Myanmar and Laos about claims of zero cases, as fears grow for their weak healthcare systems and fragile economies.
Myanmar, with 54 million people, is the world's largest country by population not to report a single case.
A doctor in Myanmar's Pathein General Hospital -- about a five-hour drive from commercial capital Yangon -- warned there is "no way" they have the resources needed to deal with the virus.
The hospital's isolation ward has only seven beds and one ventilator, said doctor Than Min Htut in a Facebook post.
"So if we have more than seven patients, where will we put them?"
Myanmar's government is urging all returning workers to self-quarantine for two weeks as state facilities are unable to handle the influx, said government-backed paper The Global New Light of Myanmar.