Health officials worldwide are warning parents about a new syndrome believed to be linked to the novel coronavirus that has infected 93 children in New York, killing three, and infected at least 100 in Europe.

The rare but dangerous illness called pediatric multisystem inflammatory syndrome can cause in children ages 2 to 15 a fever of 101 F (38 C) for five days, swollen neck glands, abdominal pain and inflammation and could require them to be treated in intensive care.

It also produces redness in the tongue, rashes, a racing heart and a change in skin color. Medical experts are alarmed, as they believed children were relatively immune to COVID-19, as so few had been infected.

President Donald Trump downplayed the syndrome at his press briefing Monday, saying, "It's a very, very, small percentage … and people recover from that."

Admiral Brett Giroir, assistant secretary for health in the US Department of Health and Human Services, said, "Most do recover, but it is serious and can be a fatal condition so we do want to make sure that parents understand that high fevers, red rashes in your child, you may want to make sure that you contact your healthcare provider. … There are treatments."

"The most important thing parents should do is err on the side of caution," Dr Howard Zucker, New York state's health commissioner, said at a press briefing Sunday.

He said that among the three children who died from the syndrome in New York, two were of elementary school age and one was an adolescent. They lived in three different counties and didn't have pre-existing conditions.

Nationwide, the illness also has been seen in New Jersey, California, Louisiana and Mississippi. Out of 93 cases currently being investigated by the New York State Health Department, 38 are in New York City.

New York Mayor Bill de Blasio said at his Sunday press briefing: "Every parent out there, if you see these symptoms, take them seriously. If you see these symptoms, report them to your doctor immediately."

Dr Oxiris Barbot, New York City's health commissioner, first warned the state's health departments to be on the lookout for the illness on May 1. On Sunday, she urged the federal government to ramp up virus testing citywide to help identify children who are at risk. "This is still evolving," she said.

New York Governor Andrew Cuomo said at his daily news briefing Friday: "So this is every parent's nightmare, right? That your child may actually be affected by this virus. But it's something we have to consider seriously now."

Cuomo added at his Monday press briefing, "No one's going to protect your children like you will."

Juliet Daly, a 12-year-old from Covington, Louisiana, had the illness. "It kind of felt like I couldn't do anything because I was in a lot of pain," she told CNN. She was airlifted to a hospital with heart failure but has since recovered.

Juliet's father, Sean Daly, said: "At first we thought she might just have the flu, but it progressed into another day. She had blue lips, her extremities were cold, so it was something very off, so we decided to take her into the doctor.

"We went to the emergency room," he continued. "They hooked her up and noticed she had a very low heart rate. That's when they diagnosed it was a heart condition or had affected her heart."