The blast, which occurred on Wednesday near the central coastal city of Zadar, killed a nine-year-old Czech child and injured three Czech adults, all of whom were in the same car.

The 46-year-old suspect -- who was travelling in the car with family and friends, including the child -- entered a prohibited area of a military training ground with the boy and allowed the latter "to take an explosive device", police said.

"The suspect then personally placed (it) in his car," they continued.

When the car subsequently broke down on a nearby road, the child passenger took the explosive device out of the vehicle.

It detonated, killing him on the spot and injuring three others in the group.

Local media said the boy was the suspect's son.

Croatian police said the man "did not comply with the clearly displayed written warnings in both Croatian and English at a marked dangerous area of a military training ground".

They said they had charged him with committing a "serious criminal act" and "endangering life and property".

An investigation is underway, including forensic tests to establish the type of explosive device, they added.

Czech authorities have confirmed the accident and the arrest.

"I can confirm that a Czech citizen has been detained in relation with Wednesday's explosion in the Zadar district.

"Considering the fact that a child died in the incident, we will not provide any information," Czech foreign ministry spokesman Daniel Drake told AFP.

The Croatian defence ministry said it regretted the incident, which it said was the first of its kind in decades.

It urged the public to abide by warning signs near military facilities, especially areas that might contain "unexploded ordnance".

Nearly 800,000 Czech tourists visited Croatia last year, heading mostly to its stunning Adriatic coast.