Dutch investigators on Tuesday raided popular taxi service app Uber's Amsterdam offices, saying it was part of a continuing probe into low-cost service UberPOP.
Transport inspectors "raided Uber's offices in Amsterdam and confiscated documents," the Dutch public prosecutor's office said in a statement.
"Uber uses the UberPOP app to book taxi rides with drivers who do not have a permit to operate as a taxi," it said, adding "it is illegal to offer a taxi service without a permit."
Prosecutors in April announced they had opened a criminal probe into Uber after Dutch judges in December banned UberPOP from taking bookings via its smartphone app.
So far Uber has been hit by 450,000 euros ($506,000) in fines for running UberPOP, Tuesday's statement said.
"It is believed that UberPOP continues to operate," and that it is therefore breaking the law, it added.
Uber uses mobile phone apps to put customers in touch with drivers who then take them where they want to go at prices lower than those of traditional taxis.
But despite its massive popularity, Uber is facing increasing limits on its activities and a barrage of legal challenges spurred on by a furious taxi lobby, who say Uber drivers should be regulated the same way as normal cabs.
Uber said it would "continue cooperating with the Dutch authorities in the investigation into UberPOP," Dutch news agency ANP said.
Uber added that the Dutch cabinet and parliament were currently working on a legal revision on the law that bans UberPOP from Dutch roads.
"We are looking at further modernising the current legislation that will lead to more jobs and give consumers a choice," Uber said.