China's aviation regulator will not yet allow Boeing's troubled 737 MAX jet to fly in the company's biggest market owing to lingering safety concerns, despite the US lifting a ban on commercial flights.

Boeing's best-selling aircraft was grounded worldwide early last year following two crashes that killed 346 passengers. 

It has since faced lengthy tests and approval processes with aviation regulators worldwide. 

But the Civil Aviation Administration of China (CAAC) said on Friday that there was "no set timetable" for the resumption of flights, according to state broadcaster CCTV, dealing a blow to the plane-making giant. 

China was the first to suspend flights of the aircraft. 

The regulator added that the results of investigations into the deadly crashes in Indonesia and Ethiopia "must be made clear" and that the aircraft design improvements must be "effective" and "receive approval".

The US Federal Aviation Administration on Wednesday approved commercial flight operations of the plane.

The crashes are believed to be linked to a faulty anti-stall system called MCAS, according to results from probes into the accidents.

Feng Zhenglin, director of the CAAC, said in October that China's prompt grounding of the aircraft was based on "zero tolerance" towards potential safety hazards. 

Boeing said last week it expects China to buy more than 8,600 new aeroplanes worth $1.4 trillion in the next two decades, increasing its forecast as domestic travel in China has recovered to pre-outbreak levels.