Spain has reported its first two deaths from acute hepatitis, or liver inflammation, among children – an illness whose causes are unknown, and it's being monitored by world health officials.

Spain's Health Ministry said that 46 cases of the illness have been reported in Spain and that three required liver transplants.

Of the three, two died – a 15-month-old baby in June and a 6-year-old in July.

The ministry said in a statement on Thursday that 507 cases had been reported in 21 European countries as of July 28, with 273 of them in Britain.

In the United States, there have been at least 180 cases and six reported deaths.

Health officials remain perplexed by the causes of the hepatitis and say the best available evidence points to a stomach bug that isn't known to cause liver problems in otherwise healthy kids.

Spain said it began closely monitoring the situation following an alert to the World Health Organization (WHO) by Britain following a spike in cases.

The ministry said that so far it has not noticed an increase in the number of cases of this type of hepatitis compared to estimates made with data from previous years.

Cases of hepatitis of unknown origin in children were first reported since the beginning of April, and over 1,010 cases had been reported by the WHO as of July 8, a month ago.