The five flights operated by China Eastern Airlines, Juneyao Airlines and Shenzhen Airlines were forced to be postponed or make emergency landings on Wednesday morning after they received telephone threats, according to the airlines.

The five flights were all bound for Shenzhen, Guangdong province.

The threats turned out to be false, and public security authorities are investigating who was behind the calls, the Civil Aviation Administration of China said in a statement on its website. It did not specify details of the threats.

The CAAC said the threats constituted the crime of fabricating and transmitting false terror information and severely affecting social order. The crime is punishable by up to 15 years in prison.

The first call came in at 7:40 am with a man claiming there was an explosive device on Shenzhen Airlines' Flight ZH9243 flying from Xi'an in Shaanxi province to Shenzhen, according to China News Service.

The man hung up before the operator at the customer service hotline could ask further information. The flight had taken off at 7:32 am from Xi'an Xianyang International Airport.

The flight then made an emergency landing in Guilin, the Guangxi Zhuang autonomous region, and passengers left the plane by using the evacuation slide in heavy security presence.

Fifteen minutes after Xi'an received the threat, the public security station at Nanjing Lukou International Airport in Jiangsu received a call from a man who said Flight ZH9866, also with Shenzhen Airlines, flying from Nanjing to Shenzhen, had a bomb aboard. He then hung up, the report said, adding the flight flew back to Nanjing.

The number of the mobile phone that made the call was registered in Shenzhen, local police told China News Service.

Shenzhen Airlines Flight ZH9889 from Beijing to Shenzhen had to postpone its takeoff from 8 am to 10:27 am after receiving a threat. All passengers were requested to leave the plane and went through security checks again. No suspicious item was found, Beijing Capital International Airport said on Sina Weibo on Wednesday morning.

China Eastern Airlines and Juneyao Airlines flights also received false threats.

Juneyao Airlines received a threat and arranged for Flight HO1111 departing from Shanghai to return to the city for inspection. Police found that there was no danger posed to the aircraft, the airline said.

The flight took off using another plane after all passengers and luggage passed safety screening, the company said.

China Eastern Airlines Flight MU2325 from Lanzhou to Shenzhen was also cleared after the airline received a similar threat.

The flight was diverted and landed safely at nearby Xi'an, according to Shanghai-based China Eastern Airlines. After a safety check, the airplane was found normal and safe.

Zhu Xu, a passenger on the Juneyao Airlines flight, said: "This is the earliest flight from Shanghai to Shenzhen, so most of us were sleeping during the flight. About 8:40 am, we suddenly realized our plane was stopped where we took off this morning - Shanghai Hongqiao International Airport."

Zhu was on a business trip to Shenzhen. According to Zhu, once the aircraft landed, flight attendants told everybody to sit still, and passengers saw many policemen and police cars, and security personnel came aboard to inspect the plane.

"We were scared and did not know what had happened. Later, we were required to go through a new round of strict security checks by loosening our belts, taking off shoes, and opening all luggage, etc," Zhu said.

Once the aircraft landed, the flight attendance told everybody to sit still, and passengers found their aircraft was surrounded by police cars and securities staff, checking something, Zhu said.

"But nobody told us what had happened until five minutes before the plane took off for the second time," he added.