The production line of a mask manufacturer in southwest China's Chongqing, March 2, 2020. (Xinhua/Wang Quanchao)

US authorities have not struck down Chinese company BYD's bid for exporting its N95 masks to the country, sources said.

The firm was told to hand in additional documents and re-submit its application by the end of May or in early June, sources close to BYD told the Global Times on Thursday. 

The comments came after the US National Institute for Occupational Safety and Health (NIOSH) said it had refused initial regulatory approval for importing the N95 masks made by BYD, the world's largest mask manufacturer in terms of capacity. 

The reasons behind the alleged rejection were not clear, though NIOSH said it had told BYD earlier this month that an on-site examination of its factories in south China yielded a "not acceptable" rating.  

BYD has received bulk orders from the US, which is battered by COVID19, including a $1 billion order with authorities in California. 

Media earlier reported that the Chinese company needs to refund $247 million that California paid for protective gear after BYD failed to meet a deadline for the masks' federal certification. In response, BYD said orders booked by the state of California have been delayed rather than cancelled, and it exports of disposable surgical masks to California have not been impacted. 

BYD's mask production capacity hit 20 million units daily in mid-April.