Boeing Dreamliner grounded by US regulator
The Federal Aviation Administration (FAA) said that airlines must demonstrate that batteries are safe before flights can resume.
Boeing said it supported the FAA but said it was confident the 787 was safe.
United Airlines is the only US airline operating the 787. Earlier, Japan's two main airlines grounded their 787s.
All Nippon Airways (ANA) grounded its fleet of 17 Dreamliners when its flight NH 692 from Yamaguchi Ube was forced to land shortly after take-off, BBC News reports.
Japan Airlines then took similar action, grounding its seven Dreamliners.
Along with United's six Dreamliners, it means well over half of all Dreamliners in operation are now grounded.
In recent weeks, Dreamliners have suffered issues including fuel leaks, a cracked cockpit window, brake problems and an electrical fire.
Finding answers
Boeing shares closed down more than 3% on Wall Street on Wednesday.
"Boeing is committed to supporting the FAA and finding answers as quickly as possible," said Boeing chief executive Jim McNerney.
"We are confident the 787 is safe and we stand behind its overall integrity. We will be taking every necessary step in the coming days to assure our customers and the travelling public of the 787's safety and to return the airplanes to service.
"Boeing deeply regrets the impact that recent events have had on the operating schedules of our customers and the inconvenience to them and their passengers."
United Airlines said it would immediately comply with the FAA's directive and would begin re-accommodating customers on alternative aircraft.
