Two letter bombs exploded on Wednesday at two separate locations in the Netherlands but nobody was hurt in the incidents, which police blamed on an extortionist who had demanded payment in bitcoin.

Both explosions were minor, one at an ABN Amro bank mail-sorting office in Amsterdam and the other 225km (140 miles) away in a mailroom of Japanese electronics group Ricoh, police said. No arrests have been made.

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"The police believe the most likely scenario is that the letter discovered on Wednesday was one of several letter bombs sent to locations across the country," they said in a statement, referring to the Amsterdam incident.

An employee in the Amsterdam sorting office heard a hissing sound as they were about to open a letter, city police said. "The employee threw the letter away and there was a small explosion," they said on Twitter.

"Payment of bitcoins is required in the extortion letter," they added.

Bitcoin is a digital currency whose payments can be difficult to trace.

ABN's chief executive, Kees Van Dijkhuizen, said he had spoken to the employee who had handled the letter at the sorting centre on the western outskirts of Amsterdam.

"Good news is that he is not wounded, bad news of course is that these things happen and that our people have to deal with it," Dijkhuizen told journalists.

The second explosion, in the southern town of Kerkrade, on the border with Germany, was at the offices of Ricoh.

"Thankfully there were no injuries, but those involved are of course very shocked," Ricoh said in a statement. The blast caused some damage and the facility was closed for forensic analysis, police said.

A third suspected letter bomb found at an ABN Amro branch in the southern city of Maastricht later turned out not to contain explosives but a computer mouse, police said.

Dutch police have been investigating a spate of letter bombs since January 3, which they said appeared to have been sent by the same person in December and January. The previous letter bombs were all intercepted before they could go off.

Previous targets have included a hotel, a gas station, a garage, a real estate agent and a bill collection service.