(NewsNation) — At least one child has died amid an increase in acute hepatitis among children, the World Health Organization said Saturday. There have been at least 169 cases reported in 12 countries and officials don’t know what’s causing the surge, that’s prompted a health advisory from both WHO and the Centers for Disease Control and Prevention.

Health authorities worldwide are investigating the mysterious increase in severe cases of hepatitis in young children. Hepatitis is inflammation of the liver, a vital organ that processes nutrients, filters the blood and helps fight infections. When the liver is inflamed or damaged, its function can be affected.

As of Thursday, WHO says acute cases of hepatitis have been reported in the United States, the United Kingdom, Spain, Israel, Denmark, Ireland, the Netherlands, Italy, Norway, France, Romania and Belgium. The largest surge is in the U.K. where 114 of the 169 cases have been reported.

The cases are being reported in children ranging from one month old to 16 years, and so far, 17 have needed liver transplants, the health organization said. WHO gave no details of the reported death.

Of the cases WHO is monitoring:

• A common cold virus known as an adenovirus was detected in at least 74 cases
• 20 of those tested revealed a COVID-19 infection
• 19 had a co-infection of COVID-19 and adenovirus

The organization said it is monitoring the situation closely and is working with British health authorities, other member states and partners.

In the U.S., Alabama health officials identified nine cases of hepatitis in children, ages 1 to 6, who also had tested positive for adenovirus since October. None of the children had COVID-19.

The CDC sent out an alert Thursday warning doctors to be on the lookout for symptoms of pediatric hepatitis, possibly linked with a cold virus, as part of a wider probe into unexplained cases of severe liver inflammation in young children.

In the advisory, the CDC asked health care providers or state public health authorities to alert the agency to any child younger than 10 who may have been diagnosed with hepatitis with an unknown cause since October.

“In November 2021, clinicians at a large children’s hospital in Alabama notified CDC of five pediatric patients with significant liver injury, including three with acute liver failure, who also tested positive for adenovirus. All children were previously healthy,” CDC officials wrote. “Case-finding efforts at this hospital identified four additional pediatric patients with hepatitis and adenovirus infection for a total of nine patients admitted from October 2021 through February 2022; all five that were sequenced had adenovirus type 41 infection identified.”

Reuters contributed to this report.