US investigators are moving swiftly and making progress in their investigation of a battery fire on a Boeing Co 787 Dreamliner in Boston this month a spokeswoman for the US National Transportation Safety Board said on Friday.

"Our investigators are moving swiftly and we are making progress " spokeswoman Kelly Nantel told Reuters after the US safety board issued a seventh update on the investigation.

All 50 Boeing 787s remain grounded as authorities in the United States  Japan and France investigate the Boston battery fire and a separate battery failure that forced a second 787 to make an emergency landing in Japan a week later.