London (CNN) A small but rising number of children are becoming ill with a rare syndrome that could be linked to coronavirus, with reported cases showing symptoms of abdominal pain, gastrointestinal symptoms and cardiac inflammation, UK health care bosses and pediatrics specialists have warned.

On Sunday, the Paediatric Intensive Care Society UK (PICS) tweeted an "urgent alert" from the National Health Service England about a small rise in the number of cases of critically ill children presenting "overlapping features of toxic shock syndrome and atypical Kawasaki disease with blood parameters" -- with some of the children testing positive for Covid-19.

The urgent alert, sent to UK general practitioners by National Health Service (NHS) England warned that over 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 UK," the Health Service Journal first reported Monday.

The alert added: "There is a growing concern that a [covid-19] related inflammatory syndrome is emerging in children in the UK, or that there may be another, as yet unidentified, infectious pathogen associated with these cases," HSJ added.

In a statement sent over the weekend to medical professionals who look after critically ill children, PICS said "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. Abdominal pain and gastrointestinal symptoms have been a common feature as has cardiac inflammation."

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