WASHINGTON (Reuters) - U.S. 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 U.S. National Transportation Safety Booard said on Friday.

"Our investigators are moving swiftly and we are making progress," spokeswoman Kelly Nantel told Reuters after the U.S. 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.

(Reporting By Andrea Shalal-Esa)