Those who choose to travel abroad despite travel advisories against doing so may face consequences, National Development Minister Lawrence Wong said yesterday.

The Government is considering measures against residents who choose to travel overseas despite border control restrictions, he said at a briefing.

"I don't think we can allow this to continue. We are discussing what additional measures may be needed," he said, adding that work pass holders who leave Singapore at this time may not be allowed to re-enter the country later.

Singapore issued a travel advisory last week telling Singaporeans to defer all travel.

Mr Wong, who co-chairs the multi-ministry task force set up to tackle the coronavirus outbreak, gave the warning as he announced that all short-term visitors will be barred from entering or transiting in Singapore from 11.59pm today.

"We don't know the reasons (why people continue to travel), it may well be that they have very exceptional reasons to travel, but we do not want to see a third wave of imported cases from returning Singaporeans," he said, pointing to the rapidly escalating virus outbreak around the world.

"We are already having to digest this wave of returning Singaporeans now. I think if we experience third or fourth recurring waves of imported cases, it will be very, very challenging," he added, referring to how almost 80 per cent of new Covid-19 cases over the past three days were imported.

Most of them were Singapore residents and long-term pass holders returning from abroad.

The Ministry of Health said these imported cases had travel histories to 22 countries. About 90 per cent of them went to GP clinics and hospitals after returning home, as they had not displayed symptoms at the checkpoints, it added.

As was announced previously, all Singapore citizens, permanent residents and long-term pass holders returning to Singapore are required to complete a 14-day stay-home notice.

Those under the stay-home notice must remain in their place of residence at all times.

"With all of these measures, which are designed to keep Singaporeans and Singapore safe... the objective cannot be achieved with government measures alone," Mr Wong said. "We do need everyone to take responsibility, to step up and do their part," he added.

Asked how long the latest border restrictions announced yesterday would be in place, Mr Wong said he could not say at this stage.

"No one will know how long this current wave of imported cases we are experiencing will last. It depends on how long the outbreak is going to continue in Europe and America, which are now the new epicentres of the virus outbreak.

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"It depends on how long more we will see returning Singaporeans... so we will monitor on a day-by-day basis," he said.

In a Facebook post last night, Mr Wong noted that the latest measures "are very major moves for a small open city like Singapore that has always been connected to the world".

"But this is an unprecedented crisis. Hence the task force has deliberated over the matter and decided to proceed with these measures," he said.

"We will continue to do everything we can to keep our borders safe, to limit the number of new cases, and importantly, to conserve our resources to focus on Singaporeans during this critical period."