Police in the Netherlands are appealing for witnesses to find a person who is planting explosives and requesting a bitcoin ransom.

According to the police

, the first attacks took place in May, when the perpetrator placed explosive devices in three different Jumbo supermarkets – a chain of stores belonging to the Van Eerd Group – in Groningen, the largest city in the north of the country.

Subsequent attacks followed in June and July when the offender sent a congratulatory card containing a small amount of explosive substance to another supermarket in Zwolle, a small town in the province of Overijssel, also in the north of the Netherlands.

The explosions have so far only resulted in property damage and the police investigation is ongoing.

Bitcoin ransoms are also increasingly being used by online hackers who hijack users' computers in exchange for payouts in the digital currency.

A senior executive in Australia was allegedly victimised after their employer was pressured by hackers into paying $14,000 in bitcoin earlier this year.

Reports that two of Hong Kong's largest banks had been targeted with distributed denial of service (DD0S) attacks by perpetrators demanding bitcoin ransoms also surfaced in May.

Dutch police image via Nessluop/Shutterstock.com