The Beijing transport authority has intensified efforts to crack down on illegal taxi services through car booking apps.

The Beijing Commission of Transport has sent 10 inspection teams this week to popular destinations including Beijing Capital International Airport to catch the cars providing taxi service without a license.

Seven private cars were found providing an illegal taxi service at the airport on Tuesday when they were charging service fees to passengers who were using smartphone apps. The apps included Yidaoyongche, Didizhuanche and Taobao. The team at the airport said four to five illegal cabs were detected on average each day.

Liang Jianwei, a deputy director of the commission, said the authority is trying to curb the rampant use of the apps to hire private cars, which are known as "black cabs".

"The use of taxi-hailing apps to hire legally registered taxis is encouraged," he added.

The number of "black cabs" in Beijing has increased sharply since the taxi-hailing apps began to appear in the middle of last year, Liang said, noting that such cabs are strictly banned under transport regulations.

All taxi drivers are required to take qualification tests, and their vehicles must be registered. Unregistered cars and unqualified drivers are prohibited from providing taxi services.

"Black cabs are unsafe for passengers, given that the physical condition of some cars could not meet the standards of Beijing taxi regulations," Liang said.

Legal taxis have full insurance coverage, while "black cabs" are often insured only with mandatory insurance, which means passengers would be compensated much less if they were injured inroad accidents while traveling in illegal cabs.