A letter bomb exploded in a Dutch bank headquarters today and another was disarmed at an IT firm as police hunt for the blackmailer behind two blasts at postal offices yesterday.

An employee of ING was rushed to hospital and treated for smoke inhalation after a 'small explosion' at the banking group's offices in Amsterdam.

Earlier, a device was found in a letter at information technology firm Unisys, in Leusden, near Utrecht, which police safely disarmed.

The latest bomb is believed to be connected to two blasts that rocked postal sorting offices in Amsterdam and Kerkrade on Wednesday morning - one targeting banking giant AMB Amro and the other hitting Japanese electronics firm Ricoh.

No injuries were reported from those explosions.

Police say the perpetrator sent ransom notes along with the bombs demanding payment in Bitcoin, or else more explosives would be sent.

The perpetrator appears to be copying the 2015 Jumbo bomber, who planted and posted explosives to Jumbo supermarkets around the Netherlands before also demanding ransom in Bitcoin.

Police officers in the lobby of the ING offices in Amsterdam after a letter bomb exploded today

Forensic investigators outside the headquarters of the ING banking group in Amsterdam where the latest in a series of letter bombs targeting businesses in the Netherlands went off

It is thought the letter was in the sorting office of Dutch bank ABM Amro when it detonated

A man identified only as Alex O, 51, was jailed for eight years for that crime in 2016 after police used DNA from a stamp that he licked to identify him.

Police sources, who spoke to Dutch site De Telegraaf about the most recent cases, said they are investigating the crimes as extortion but refused to reveal how much had been demanded in ransom.

Wednesday's explosions come a month after letter bombs were posted to addresses in Amsterdam, Rotterdam, Utrecht and Maastricht.

Police are probing whether the most recent attacks are linked to the attempted bombings last month.

An employee in the Amsterdam sorting office heard a hissing sound as they were about to open a letter, the city's police said.

'The employee threw the letter away and there was a small explosion,' the force added on Twitter.

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

'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.

'Thankfully there were no injuries, but those involved are of course very shocked,' Ricoh said in a statement cited by Dutch media.

It caused some damage, and the facility was closed for forensic analysis, police said.

A letter bomb detonated in the postal office of a building on a business park in Amsterdam around 8am Wednesday, with no injuries reported

A second device then exploded a short time later on this business park in the city of Kerkrade, around 120 miles south of Amsterdam

The bomb is thought to have exploded in a mail sorting office in Kerkrade, though it is unclear if the premises was deliberately targeted

The blasts come a month after another series of mail bombs, purporting to be from Dutch debt collection agency CIB, were sent out to business in Amsterdam, Utrecht and Rotterdam

The Okura Hotel and a petrol station in Amsterdam, a real estate agency in Utrecht, and a petrol station and car showroom in Rotterdam were all previously targeted.

The sender of the packages was listed as the head offices of a debt collection company called CIB, located in Rotterdam.

Police previously said that the CIB mail labels appeared to have been faked, and that the CIB was itself targeted.

The packages, none of which exploded, were followed by a warning letter which Dutch police have not released.

Detectives said at the time that they believe the bombs were sent by a single individual and that, while the motive was unclear, it could involve blackmail.

No arrests have been made over the January bombs.

Investigations into Wednesday's blasts are in the early stages, and police have not officially linked them together.

Officers were also unable to say whether the fresh blasts were linked to the bombs sent out last month.