Thailand now faces ending up like Italy with a soaring number of Covid-19 patients, or becoming like Japan which has been able to contain the spread of the virus, an expert has warned.

As Thailand recorded 122 new Covid-19 cases, raising the total to 721 as of Monday, Dr Prasit Watanapa, dean of the faculty of medicine at Siriraj Hospital, Mahidol University, said people must stay home and stop socialising or Thailand risked ending up like Italy.

He blamed people who visit virus-risk venues such as boxing stadiums and pubs, and refuse to self-isolate.

Countries that failed to control the disease saw Covid-19 cases rise from 100 to 200 in three days. That increase in Thailand took 3.5 days, Dr Prasit said.

The number of Covid-19 cases in Thailand was rising by 33% daily, similar to Germany. At this rate, the number of infections would reach 351,948 by April 15, Dr Prasit said.

Of them, 52,792 would be hospitalised, and the number of critically-ill people in ICUs would be 17,597, and that would exceed the country's medical resources, Dr Prasit said.

Deaths would then total 7,039 by April 15, he warned.

Countries and territories that have been able to control Covid-19 were Japan, Taiwan, Hong Kong and Singapore, he said.

To keep the number of Covid-19 patients within the country's capacity to treat them, the increase in local cases must be limited to 20% daily. If this can be achieved, the number of infections would be only 24,269 by April 15, Dr Prasit added.

"To achieve this, people must stay at home, do not socialise... If you must go out to buy food, you must wear a mask, go the shortest distance and return home as fast as possible," Dr Prasit said.

"Talking normally to another person produces droplets that travel about one metre. So people must stay at least two metres apart. When you are talking to people, you must wear a mask. Wash your masks daily," he said.

"A communicable disease cannot be transmitted if people do not make contact. Home is the safest place, unless there is an infected person."

With strict social distancing, the number of new cases would start to fall in four weeks, he said.