Two letter bombs exploded at offices in the Netherlands on Wednesday without causing casualties, in what police said was an extortion attempt targeting businesses.

The devices went off in the mailrooms of an ABN AMRO bank office in the capital Amsterdam and an office of the Japanese copier firm Ricoh in the town of eastern Kerkrade, near the German border.

The sender had demanded an amount in the crypto-currency bitcoins and threatened to send further explosive packages if payment was not made, the Dutch news agency ANP reported.

A police spokesman said an employee at ABN AMRO "heard a hissing sound" coming from a letter and threw it away "after which a slight bang occurred."

Ricoh said in a statement that "fortunately there were no injuries, but the people involved were of course very shocked."

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.

Police were also investigating whether there was a link to at least seven letter bombs sent to seemingly random targets in the Netherlands in late December and early January.

The targets then included hotels, service stations and the central debt collecting agency.