Police officers monitor the sky for drones that might interfere with aircraft at a checkpoint near Chengdu Shuangliu International Airport, in Chengdu, Sichuan province, on Saturday.Li Zhi / For China Daily

Police in Chengdu, Sichuan province, called for people to report illegal drones whenever they spot one flying in the clearance protection zone of the city's airport.

An announcement by the city's public security bureau on Monday encourages people to report at any time around the clock, and pledges to hold those flying drones illegally to account.

The clearance protection zone of Chengdu Shuangliu International Airport refers to 10 kilometers from the center of its runways on both sides and 20 km from the ends of the runways.

The announcement, which is effective for one year, came after illegal drones were found flying on Saturday, forcing 10 flights to land at alternate airports.

It is not the first time illegal drones have showed up at the airport. Seven cases of drones flying without a permit took place in April, causing flights to be diverted, returned or canceled, according to Lu Junming, an information officer at the airport.

At 2:38 pm on April 21, four drones flew in the clearance protection zone of the airport, and one passed below an aircraft in the process of landing. They forced 58 flights to land at alternate airports, four to return to their point of origin and at least 10 to be canceled.

More than 10,000 passengers were stranded in the Chengdu airport, which did not resume normal operations until early the next morning, Lu said.

On Monday, a drone in the protected airspace of an airport in Kunming, Yunnan province, forced 28 flights to land at alternate airports and four others to return to their starting point on Monday.

Sources at Kunming Changshui International Airport said the drone was spotted at about 2 pm by crew members aboard a plane that was on its landing approach. The airport resumed normal operations after 3 pm.

Known as "bombs in the air", illegal drones pose a safety hazard to civil aviation. Measures have been taken by police in the past two weeks to crack down on illegal drones flying around Chengdu airport, including a reward for anyone with information.

The Sichuan provincial department of public security announced on April 20 that the reward would be raised from at least 1,000 yuan to 10,000 yuan ($145 to $1,450) if the facts are verified.

On April 28, police set up 91 monitoring stations deploying people with telescopes in the protected zone.

Residents in the zone say the illegal drone flying has gone on for a long time, endangering locals, too.

A farmer surnamed Yang told how his 10-year-old son was knocked down by a drone three months ago, which hurt one of his ears. Another farmer, surnamed Zhang, was hit by a drone just more than 10 days ago.

"I was angry and trampled the drone with my feet," Zhang said.

Yang said drone fans come in threes and fives to a vast expanse of land 3 kilometers northwest of the airport to fly drones on weekends.

Wang Mingwen, a professor of law at Xichang College in Sichuan, suggested regulations should be made to monitor drones.

Each drone has to be registered with its owner's identification card number so that the owner can be found if it flies illegally, she said.

huangzhiling@chinadaily.com.cn