China Eastern Airlines has announced that it would penalize pilots involved in staging flight returns over labor grievances, the company said.

The Monday staged 18 flight returns to protest pay and working conditions, China's state-run Xinhua news agency reported. Among other complaints, pilots were angered by being required to sign lifetime contracts with state-owned airlines.

The airline, based in Shanghai said poor weather was a factor in the flight returns, the news service reported.

"We are concerned about public suspicions (that weather was not the real cause) and an investigation is under way," the company said in a statement.

A spokesman for the Civil Aviation Administration of China said the agency would ask authorities to impose penalties on the pilots if they disrupted flights on purpose.