''All the lions remain bright, alert and are eating well,'' the Channel News Asia report quoted Dr Sonja Luz, vice president of conservation, research and veterinary at Mandai Wildlife Group, the zoo operator, as saying.

Meanwhile, the country also reported 3,397 fresh COVID-19 cases, including 169 from dormitories for migrant workers. With this, the total number of cases in the country has increased to 2,24,200.

The Ministry of Health reported 12 deaths due to coronavirus-related complications that pushed the death toll to 523.

According to the CNA report, the four Asiatic lions tested positive at the Night Safari, a popular tourist attraction. The lions had exhibited mild symptoms including coughing, sneezing and lethargy on Saturday.

''This was upon exposure to staff from the Mandai Wildlife Group who tested positive for COVID-19,'' the report quoted the Animal and Veterinary Service (AVS) as saying.

The AVS has asked the Mandai Wildlife Group to isolate all nine Asiatic lions and five African lions in their respective dens.

The AVS used PCR tests to diagnose the four Asiatic lions.

An African lion at the Singapore Zoo also showed symptoms on Monday and is undergoing testing.

The Mandai Wildlife Group said that three keepers from the Night Safari Carnivore section have tested positive for COVID-19.

Two keepers had initially ''tested positive while off-duty'', the CNA quoted the Mandai Wildlife Group as saying.

Tests were conducted on team members who had been in contact with the two keepers. A third employee, who was asymptomatic, subsequently tested positive at work and was stood down from duty.

''They received (a) confirmatory positive PCR test on November 8,'' the zoo operator said.

In a separate statement, the Mandai Wildlife Group said that the Asiatic lion exhibit along the tram route at the Night Safari has been closed since Sunday after the four lions showed respiratory symptoms.

''There has been evidence that animals, in general, do not fall seriously ill from the virus.

''We expect that the lions will make full recovery with minor supportive treatment. However, anti-inflammatories and antibiotics may be prescribed if further treatment is needed,'' said Dr Luz of the Mandai Wildlife Group.

Of the 3,397 fresh cases reported in the country, six were people who arrived from abroad, the Ministry of Health said.

Sixty-eight patients are unstable and being closely monitored in intensive care units (ICUs), while 72 are critically ill and intubated.

A total of 1,700 cases are warded in hospitals, with 300 patients requiring oxygen supplementation in general wards, the CNA reported.

Several inmates and staff of Institution A1 of Changi Prison Complex were among those who tested positive for COVID-19 on Tuesday, according to the Singapore Prison Service (SPS).

Affected housing units have been locked down to prevent further spread of the virus, The Straits Times quoted the SPS as saying.

This follows the confirmation earlier on Tuesday that Malaysian drug trafficker Nagaenthran K Dharmalingam, who is facing execution, had tested positive for COVID-19. He is housed in Institution A1.

SPS said the Indian-origin Malaysian, who chose not to get vaccinated, had transient contact with two staff who had tested positive.

Nagaenthran is ''well and asymptomatic'' and did not report any illness prior to his court hearing, the SPS said in a statement late on Tuesday night.

As part of safe management measures, the SPS said, staff and partners entering prison facilities must undergo antigen rapid tests (ART) twice weekly, while external personnel entering prisons on an ad-hoc basis are required to undergo an ART prior to each entry into prisons.

As of Sunday, 97 per cent of the eligible inmates had received at least one dose of the COVID-19 vaccine and 90 per cent had completed both doses, it said.

It added that 99.5 per cent of the staff have taken both doses.