The UN body said it was aware of 169 rare cases of acute hepatitis in children as young as one month old, with cases reported in 12 countries as of April 21.
The majority of the cases have been detected in the UK, which has recorded 114 cases. Severe cases of the condition, an inflammation of the liver, is highly unusual in young children.
The outbreak has also spread to Spain, Israel, the United States, Denmark, Ireland, The Netherlands, Italy, Norway, France, Romania, and Belgium.
The outbreak, whose cause is currently unknown, was first detected in Scotland earlier this month with cases dating back to January.
The WHO has given no details of the death and has not said where it occurred.
In a briefing, the WHO said: “The United Kingdom first reported an unexpected significant increase in cases of severe acute hepatitis of unknown origin in young, generally previously healthy children.
“An unexpected increase of such cases has now been reported by several other countries – notably Ireland and the Netherlands.”
Scientists have suggested that adenovirus, a common type of virus that can cause common colds, could be behind the number of cases of severe hepatitis.
However the briefing said the cause was still currently unexplained, adding: “While adenovirus is currently one hypothesis as the underlying cause, it does not fully explain the severity of the clinical picture.”
In an interview with the US health website, Stat, the WHO’s Richard Pebody urged countries to check cases.
“Although the numbers aren’t big, the consequences have been quite severe,” he said. “It’s important that countries look.”
The UK Health Security Agency has urged parents to be vigilant to the symptoms of hepatitis.
The UKHSA’s Dr Meera Chand said officials were working to “investigate a wide range of possible factors which may be causing children to be admitted to hospital with liver inflammation known as hepatitis.”
“One of the possible causes that we are investigating is that this is linked to adenovirus infection. However, we are thoroughly investigating other potential causes,” she said.
Hepatitis symptoms include: dark urine, pale, grey-coloured poo, itchy skin, yellowing of the eyes and skin (jaundice), muscle and joint pain, a high temperature, feeling and being sick, feeling unusually tired all the time, loss of appetite, tummy pain.