Boeing’s BA.N plane deliveries to China have been delayed in recent weeks due to a Chinese regulatory review of batteries related to the cockpit voice recorder, a source familiar with the matter told Reuters.

The U.S. planemaker said in a statement it is working with Chinese customers on the timing of their deliveries pending a review by the Civil Aviation Administration of China of batteries contained within the 25-hour cockpit voice recorder.

In a year-end 2023 filing, Boeing said it had approximately 140 737-8 aircraft in inventory, including 85 aircraft for customers in China. Boeing has delivered 22 aircraft to China in 2024 through the end of April.

Boeing said the Federal Aviation Administration has certified this system, which stores more data than past cockpit voice recorders.

China in March Boeing gave permission last month to resume deliveries of its 737 MAX 8 to local customers, ending an import freeze on the U.S. planemaker’s most profitable jets which came shortly after its first delivery in 2019.

China was the first country to ground MAX jets after two accidents in 2018 and 2019 that killed nearly 350 people.