Two of the new patients, all of whom were Thai nationals, had returned from vacation in Japan's northern island of Hokkaido and came into contact with the third patient, an 8-year old boy, said Sukhum Kanchanapimai, permanent secretary at the ministry.

The male patient had not disclosed his travel history when he first sought medical attention and risked becoming a super-spreader, an incident of large transmission from a single source, health minister Anutin Charnvirakul said.

"We have to criticize this case ... it creates a lot of trouble for us," Anutin said.

The patient came to the hospital on Feb. 23, but did not disclose his travel history until the next day, B Care Medical Center said in a statement.

The patient came into contact with 30 medical personnel at the hospital, who later tested negative for the virus and are in self-quarantine at home, the statement said.

Passengers, cabin crew and students in the boy's class were all being tested, officials said.

Public Health Ministry officials will be prohibited from traveling to risky countries. If travel is required, personnel are required to self-quarantine for 14 days, Sukhum said.

At-risk countries include China, Macau, Hong Kong, Taiwan, South Korea, Singapore, Italy, Iran and Japan.

"For the airlines, please reduce promotions," Anutin said.

"Even though tickets are cheap, it could be your last holiday."

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