Spain's leftist government on Wednesday opened an investigation into holiday rental platform Airbnb for allegedly failing to take down adverts for thousands of apartments without a tourist licence.

Mass tourism is a hot topic in Spain, the world's second most-visited country, as foreign visitors fill the coffers of businesses but fuel local anger about unaffordable housing.

The consumer rights authority is investigating a "tourist apartment rental platform" after previous warnings about the adverts went unheeded for several months, the government said in a statement without naming Airbnb.

A source close to the investigation and the California-based company told AFP the probe targeted Airbnb.

The social rights and consumer ministry said the failure to comply was punishable by fines of up to 100,000 euros ($104,000) or "four to six times the illicit profit made".

Airbnb said it would appeal against the investigation and "always asks hosts to confirm they have the necessary permits and comply with local regulations before advertising themselves on the platform".

The ministry has no authority to enforce tourist accommodation rules and used "an indiscriminate method" to draw up a list including properties that may not need licences, an Airbnb spokesperson told AFP.

Tourist magnet Barcelona and the popular southern coastal city of Malaga have announced measures to clamp down on short-term tourist rentals in a bid to tame popular discontent at rampant housing prices.

The government has encouraged such efforts and is working on creating a register of tourist apartments to tackle illegal rentals and strengthen regulation.

"The uncontrolled tourist apartment sector often operates illegally, without a licence... we have to ensure no company, however large or powerful it may be, is above the law," said social rights minister Pablo Bustinduy.