Bangladesh said Friday the Uber ride-sharing service was operating "illegally", just days after it launched in the capital.

A notice placed in a high-circulation newspaper by the Bangladesh Road Transport Authority told drivers not to work through the popular phone app.

"Uber, the online-based taxi service, is being operated completely illegally," said BRTA director Mohammad Nurul Islam in the advertisement.

There was no immediate comment from Uber, which launched its on-demand car ride services in Dhaka on Tuesday saying it would help reduce traffic in one of the world's most congested cities by encouraging ride-sharing.

Uber appeared to have government backing when a junior minister said the launch was "part of our efforts to build smart cities".

The service has revolutionised car share rides across the globe since it was launched in 2010, becoming one of the world's most valuable startups.

But it has faced protests from established taxi operators in many places and been hit by a series of lawsuits and regulatory hurdles.

BRTA operations director Sitangshu Shekhar Biswas told AFP there was no "legal framework" for Uber to operate in Bangladesh.

"It can tie up with the country's two existing taxicab companies because we've fixed their rents and other guidelines. But its app cannot be used in other private cars or vehicles," he said.