China Eastern Airlines said on Monday it has grounded pilots who flew domestic flights back to their departure airports last week in what local media said was a strike over pay and work conditions.

Strikes are unusual in China, where there is only one legal, government-backed union.

China Eastern, acknowledging for the first time that bad weather was not entirely to blame for the return of 21 flights operated by a subsidiary to their departure airports in southern Yunnan province, said it would take strict measures in response to the incident once an investigation was complete.

"There are some flights that, rather than returning for weather-related reasons as the crew indicated at the time, clearly had a human element involved," the statement said, referring to disrupted flights on March 31 and April 1.

"China Eastern's Yunnan subsidiary has already temporarily grounded those individuals suspected of involvement in the return of flights for contrived reasons," it added.

Two senior executives at the unit were suspended from duty and China Eastern deputy general manager Li Yangmin has been appointed as the unit's new chief, the statement added.

Local media said last week the pilots were upset that they were limited to domestic routes, with shorter flying hours and lower pay than counterparts in the parent company. They were further discouraged to find they would be taxed heavily on overtime pay.

China Eastern representatives could not be reached on Monday but they repeatedly said last week that flights that had taken off from Yunnan's capital, Kunming, and returned did so merely because of poor weather.

Chinese airlines are increasingly facing a shortage of pilots due to booming air travel and the emergence of private airlines, such as budget carrier Spring Airlines, which are offering higher wages to lure pilots from big state-run firms.

Pilots at state carriers often must make sizable payments to their employers if they quit their jobs.

Last month, pilots for Shanghai Airlines and newly formed Wuhan East Star Airline staged coordinated "sick-ins" in two separate incidents, local media reported.

The mid-air strike by China Eastern pilots drew heavy coverage in the Chinese media, in a sign of concern just four months before the opening of the Beijing Olympics.

China's aviation regulator said in a statement over the weekend that it was investigating the return of the China Eastern flights and warned that pilots would be punished severely if their actions were found to be improper.