China’s civil aviation regulator initiated a review of design changes submitted by Boeing Co. to its 737 MAX aircraft after the country grounded the fleet last month over safety concerns.

The Civil Aviation Administration of China (CAAC) said Tuesday at a regular news briefing that the administration received Boeing’s application April 2 for an airworthiness certificate. The application came via the U.S. Federal Aviation Administration (FAA), which sent Boeing’s applications to nine countries, according to CAAC.

CAAC has conducted a review and responded to Boeing and the FAA on issues regarding the aircraft's airworthiness. A CAAC official said the administration is waiting for their response without giving a timeframe for a resumption of flights.

China was the first country to ground the newest version of Boeing's 737 model last month after a deadly Ethiopian Airlines crash. The country’s airlines operate the world’s biggest fleet of 737 MAX planes.

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