British health officials have warned doctors about a "serious coronavirus-related syndrome" affecting children, according to Britain's Health Service Journal (HSJ).

An alert sent to general practitioners across the country, which the HSJ claims it has seen, said that: "In the last three weeks, there has been an apparent rise in the number of children of all ages presenting with a multisystem inflammatory state requiring intensive care across London and also in other regions of the U.K."

An article published by the HSJ said the note underlines "a growing concern that a (COVID-19) related inflammatory syndrome is emerging in children in the U.K., or that there may be another, as yet unidentified, infectious pathogen associated with these cases."

The HSJ said the absolute number of children affected so far is thought to be small, but little is known so far about the issue.

The syndrome has the characteristics of serious COVID-19, "but there have otherwise been relatively few cases of serious effects or deaths from coronavirus in children."

"Some of the children have tested positive for COVID-19, and some appear to have had the virus in the past, but some have not," it said, adding that many fewer children than adults have had a serious illness with the virus or died.

The alert was sent to doctors more widely and is confirmed in a separate "urgent alert" issued last night by the Pediatric Intensive Care Society.

"The cases have in common overlapping features of toxic shock syndrome and atypical Kawasaki Disease with blood parameters consistent with severe COVID-19 in children," the note said.

The number of deaths in the coronavirus outbreak in the U.K. is nearly 21,000, but most of the victims are over 60 years old, according to official figures.

A 5-year-old child who died after testing positive for coronavirus is the youngest victim in the U.K., according to the National Health Service England.