TAIPEI (AFP) - Taiwan has fined Uber a total of TW$32.75 million (S$1.4 million) since last September for improper registration, turning up the pressure on the app-based taxi service as authorities mull revoking its license to operate on the island.

The highways department has handed down 243 fines totalling TW$32.75 million since last September, when penalties on Uber first started for operating without proper registration.

It is the latest setback for the company, which faces regulatory roadblocks in several other countries, after five of its drivers were arrested in Hong Kong earlier this week.

The San Francisco-based service has been entangled in disputes with Taiwanese authorities since it launched in Taipei in 2013.

The authorities say Uber has registered with the government as a company but has not declared itself as a transport business - a classification that the company refutes, claiming it is just a platform that connects drivers and passengers.

"They haven't registered as a transportation business but they fit into our definition of one," Mr Liang Guo-guo, spokesman for the directorate general of highways, told AFP on Friday. "So when they carry passengers, it's a violation."

The authorities "have been discussing eliminating their licence", Mr Liang said. "But there are some legal and operation rights issues."

Uber did not immediately respond to e-mailed queries about the penalties and the risk of being kicked out of the Taipei market.

The authorities also separately fined Uber drivers 251 tickets totalling TW$10.87 million for illegally carrying passengers.

Hong Kong police raided Uber's office and arrested five drivers in a sting operation earlier this week on the basis that they were "illegally driving a car for rental purpose and driving without third-party insurance".

The incident sparked a backlash from the public, with more than 46,000 people signing on a petition since Thursday to support Uber's operations in the southern Chinese city.

The service is also facing legal challenges in the US state of California over how its drivers are classified.

Last month it suspended its UberPOP service in France following a spate of violent protests sparked by angered taxi drivers.

Despite the setbacks, a recent capital raising of nearly US$1 billion (S$1.4 billion) would put Uber's value at more than US$50 billion, the Wall Street Journal reported.