For the past two days, dozens of airplanes scheduled to fly to the Chinese city of Chengdu have had to land at alternative airports after drones were sighted flying in the clearance protection zone.

According to reports from the Chengdu-based newspaper, the Cover, 11 flights to Chengdu, the capital city of southwest China's Sichuan Province,were disrupted on Monday. A further 19 flights either landed in other nearby cities, including Chongqing and Guiyang, or returned the next day.

Sichuan police are investigating the cases, but Wang Ya'nan, Deputy Editor-in-Chief of the monthly magazine Aerospace Knowledge, said it is a global headache to arrest the people involved under current circumstances.

"The remote controllers and the drones are often not in the same place. So the operators may have already left when the drones enter the restricted area."

The flying of drones close to the airport could pose a great threat to aviation safety as the drones could easily be sucked into the engine or crash into an airplane's cockpit window.

Such incidents are not rare in China, as numerous reports have occured in Beijing, Shenzhen, Kunming, and other cities across the country over the past few years.

China's drone market has been experiencing a boost lately, with a compound annual growth rate of 122.4 percent, according to market research firm International Data Corp. China's civil aerial photography drone shipments are expected to reach 5.5 million by 2020, with consumer level drones exceeding 3 million.

At the same time, Chinese authorities have been cracking down on the illegal use of civil unmanned aircraft and beefed up guidelines and regulations to increase air safety, which may lead to criminal charges if violated.

In Sichuan, drone users are required to have real-name registration, and people who report illegal flying of drones to the police can receive rewards of no less than 1,000 yuan (about US$145).

Drone enthusiasts are advised to raise their awareness of safety issues. Some experts have suggested regulating drones by attaching GPS systems to their flying control systems, as well as installing defense system at airports to keep flying machines away.