U.S. Transportation Secretary Elaine Chao warned regulators on Wednesday against sending conflicting messages as they work to ensure Boeing Co's 737 MAX is safe to resume operations.

Boeing's best-selling jet was grounded globally in March, days after the crash of an Ethiopian Airlines flight that followed a similar Lion Air disaster in Indonesia in October. The two crashes took 346 lives.

"International coordination is important in all aspects of this process," said Chao. "The traveling public will not be well served if there are conflicting signals given by different regulatory authorities around the world."

Airlines have urged global regulators to coordinate with one another to avoid damaging splits over safety as they evaluate software changes undertaken by Boeing. Some countries have already vowed to run their own independent validation studies before restoring flights.

On Monday, the Federal Aviation Administration met with global regulators to discuss steps needed to return the jet to flight.

Wednesday's comments came as aviation officials from around the world gathered in Montreal for the International Civil Aviation Organization's assembly, held every three years. Chao spoke on the floor of the assembly, alongside many other countries.