The U.S. FAA has grounded the six Boeing 787-8s operated by United Airlines, the only U.S. carrier flying the type to date, as problems with the aircraft’s lithium ion batteries continue to dog the in-service fleet.

“As a result of an inflight Boeing 787 battery incident [on Jan. 16] in Japan, the FAA will issue an emergency airworthiness directive (AD) to address a potential battery fire risk in the 787 and require operators to temporarily cease operations,” the agency says in a press release, issued Wednesday evening in Washington. “Before further flight, operators of U.S.-registered, Boeing 787 aircraft must demonstrate to the [FAA] that the batteries are safe.”

Today’s incident involved an All Nippon Airways 787 that declared an emergency and diverted due to “multiple messages in the cockpit concerning the battery and other systems that were reported,” says the U.S. National Transportation Safety Board, which has dispatched an investigator to Japan to analyze the aircraft. The ANA crew reported an odor in the cabin and cockpit.

The incident followed an earlier 787 battery incident that occurred on a Japan Airlines 787 on the ground in Boston on Jan. 7, also under investigation by the NTSB. Both battery failures “resulted in release of flammable electrolytes, heat damage, and smoke,” says the FAA.

“The root cause of these failures is currently under investigation,” the FAA says. “These conditions, if not corrected, could result in damage to critical systems and structures, and the potential for fire in the electrical compartment.”

Both ANA and Japan Airlines have voluntarily grounded their fleets.

The FAA on Jan. 11 announced a “comprehensive review” of the 787’s “critical systems,” and says there may be “further action pending new data and information.”

“In addition to the continuing review of the aircraft’s design, manufacture and assembly, the agency also will validate that 787 batteries and the battery system on the aircraft are in compliance with the special condition the agency issued as part of the aircraft’s certification,” the FAA says.