China’s aviation regulator said on Monday its major safety concerns with the Boeing Co 737 MAX had to be “properly addressed” before conducting flight tests but it was studying a plan with U.S. planemaker for clearing aircraft to fly.

The Civil Aviation Administration of China (CAAC) conducted comprehensive and in-depth technical scrutiny of the Boeing 737 MAX, the agency’s vice head Dong Zhiyi said, giving the regulator’s stance on the plane which China grounded in early 2019.

The U.S. and several other regulators have already cleared the plane to fly again.

-We’ll conduct flight tests in a planned and step-by-step way once our major safety concerns are properly addressed- Dong said, adding that cooperation between FAA and Boeing had been positive.

China was the first country to ground the 737 MAX in March 2019 following two fatal crashes. At the time, more of the planes had been operating in China than anywhere else in the world. China makes up about a quarter of MAX sales.

The CAAC has outlined three principles for the jet to return to service, including certified design changes, proper training for the pilots and specific findings into the crashes.

The U.S. Federal Aviation Administration (FAA) lifted its flight ban on the aircraft in January. It was followed by the European Union Aviation Safety Agency (EASA) and several others.

Australia said on Friday it would lift a ban on flights.