NEW YORK - Fifteen children in New York City (NYC) have been hospitalized with a rare syndrome possibly connected to COVID-19, officials said on Tuesday.

The children have shown a multi-system inflammatory syndrome with persistent fever, rash, abdominal pain or vomiting. Four of them tested positive to coronavirus and six others had antibodies, said NYC Mayor Bill de Blasio at his daily briefing.

"Even though it's uncommon compared to the hundreds of thousands of people who have contracted, this disease is still causing us concern," he said.

NYC Health Commissioner Oxiris Barbot said that Boston and Philadelphia have also found a few similar cases.

"We're not sure what to make of this yet. We're still learning every day about how COVID-19 behaves," said Barbot.

The city's health department has instructed all health care providers to report any patients under 21 years old with these symptoms immediately.

The department said in a statement that the patients' symptoms are associated with toxic shock or Kawasaki disease, a rare autoimmune sickness that causes the walls of arteries to become inflamed.

No fatalities have been reported yet, while over half of the children required blood pressure support and five required mechanical ventilation, according to the statement.

Similar cases have been recently reported by authorities in the United Kingdom, the department said.

New York City has over 176,000 COVID-19 cases by Tuesday evening, according to official data.