Experts have called for better airport security checks and harsher punishment for people who make false threats after hoax calls to airlines delayed two flights and forced another to make an emergency landing this week.

Two Air China flights received threatening phone calls before takeoff on Tuesday, the flagship carrier said in a statement posted on its official account on micro-blogging site Sina Weibo.

The flights, CA4111, from Lhasa to Beijing, and CA1578, flying from Nanchang to Beijing, were threatened in phone calls from unidentified people. The calls prompted the planes to undergo strict checks according to civil aviation management regulations, the announcement said.

Air China said no abnormalities were found on either of the aircraft. In an online post late Tuesday evening, the airline wrote that passengers were boarding the aircraft and preparing for takeoff.

On Monday a passenger plane with 196 people on board operated by China Southern Airlines made a forced landing in Northwest China's Gansu province after receiving an anonymous terrorist threat.

The plane, CZ680, traveling from Istanbul to Beijing via Urumqi, made a forced landing in Gansu Zhongchuan Airport hours after it took off from Urumqi, capital of the Xinjiang Uygur autonomous region in Northwest China.

After strict checks, no danger was found and the plane took off five hours after landing. According to police, a suspect, surnamed Wang, has been detained. Police said Wang admitted fabricating the fake threats. All other information about the suspect and the motivation for the threats remain unknown to the public.

"We have seen anonymous calls made to airlines before, but it is quite extraordinary to see three such incidents happening within two days," said Li Wei, an expert in security and arms control research at the China Institutes of Contemporary International Relations.

"It's possible that the threatening calls (on Monday and Tuesday) may be related to, and carried out by, an organized gang for some purpose," he said. "We must now await the results of the investigation into the incidents."

Anonymous threatening calls are a cheap means of terrorist threat and create trouble for the whole of the international aviation industry, he said.

"Although many anonymous calls such as these result in no serious consequences, they still pose a security threat to aviation and leave a psychological scar on the passengers," Li said.

Li suggested airports improve the reliability of security checks to deal with similar threats.

"Another method is to severely punish those making fake threats, so people who want to make them, for whatever reason, will be deterred," he said.

Hong Daode, a law expert at Chinese University of Political Science and Law, agreed.

"We should make the public aware that all groundless threatening calls made to passenger planes are a violation of law and will be punished," he said.

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