SINGAPORE - Any group seen gathering in public will immediately be issued a written warning by enforcement officers, said Prime Minister Lee Hsien Loong.

He warned that the authorities will strictly enforce the "circuit breaker" measures from Thursday (April 9), noting there are still far too many public gatherings taking place.

There were still crowds seen in nature reserves, markets and other places, on the third day of strict measures that include closing most workplaces and barring social gatherings of any size in homes or public spaces like parks.

Those caught flouting the rules for the first time will be issued a stern written warning, said Minister for the Environment and Water Resources Masagos Zulkifli in a Facebook post on Thursday.

Second-time offenders will be fined $300, and those nabbed a third time will be charged in court.

In a Facebook post, PM Lee said the number of new Covid-19 cases is increasing sharply, and that people must comply with stay home measures "very strictly".

Singapore reported 142 new coronavirus cases on Wednesday, taking the total number of reported cases here to 1,623.

Said PM Lee: "If your loved ones do not understand how serious this is, please try hard to help them understand. The more we take liberties with the stay home measures, the longer these painful measures will have to last.

"I know we all want to go back to normalcy, but this can only happen if we take things seriously," he stressed.

Under the circuit breaker measures that kicked in on Tuesday until May 4, people have to stay home and step out only for essential tasks, such as going to work if they are in essential services, buying food and groceries, or for a short bout of exercise.

Eating outside is prohibited. A 71-year-old man was arrested on Tuesday for disorderly behaviour after he insisted on eating at a void deck.

Some 3,000 advisories were issued to those flouting the elevated safe distancing rules on Wednesday, in addition to the more than 7,000 the day before.

Mr Masagos said enforcement officers will immediately take down the particulars of anyone found in breach of the stricter safe distancing measures. Officers had previously issued a verbal warning before allowing people to disperse.

Said Mr Masagos: "We need everyone's cooperation for these circuit breaker measures to be effective. There are still many people who are not taking the Covid-19 situation seriously.

"Some are even uncooperative, insisting on dining in at eating places, not maintaining a safe distance when queueing in markets, and gathering in parks to eat or exercise together."

He urged everyone to "make the right choice", stay home and to stay at least 1m apart.

"Remind your family members, especially the elderly, to stay at home. They are the most vulnerable, and we need to protect them. Remind also the young, who are out of school during this period, to meet their friends online, and not gather in person. Young or old - none of us are immune."

Earlier on Thursday, the National Parks Board had begun deploying drones to monitor crowds at Bukit Timah Nature Reserve.

The Land Transport Authority said standing spaces and seats will be marked out on public transport and stations to make sure commuters maintain safe distancing.

More than 100 transport ambassadors are expected to help regulate passengers and make sure rules are followed in the coming weeks.

In a subsequent Facebook post, PM Lee said he spoke on the phone with his New Zealand counterpart Jacinda Ardern on Thursday afternoon.

They updated each other on their management of Covid-19, he added. "PM Ardern and I also discussed how we can support each other during this time, including keeping supply chains intact between the two countries."

PM Lee said Ms Ardern shared that New Zealand only saw its coronavirus cases start to come down on day 11, after it barred residents from leaving their home except for essential supplies or exercise.

"We are only in Day 3 and still have a long way to go. If we all comply strictly, as the New Zealanders have done, hopefully by our Day 11 we too will see positive results," he said.

"We cannot wait to save lives. Please do your part today."