US safety regulators are still seeking what exactly caused a battery fire on a Japan Airlines Boeing 787 plane, according to an update on their investigation.

The National Transportation Safety Board on Thursday released an interim report packed with technical details about the lithium-ion battery fire on the JAL plane parked at Boston's Logan Airport.

But the report on the initial findings of the NTSB investigation offered no pinpointed cause of the fire that contributed to the global grounding of 787s almost two months ago.

"The NTSB's investigation into the probable cause of the 787 battery fire at BOS is continuing. The NTSB is also continuing to review the design, certification, and manufacturing processes for the 787 lithium-ion battery system," the independent government agency said in the report.

In its last briefing a month ago, the NTSB identified a short circuit on a single cell in the eight-cell JAL battery that had sparked overheating.

Another burned battery incident, on an All Nippon Airways 787 that was forced into an emergency landing in Japan, led regulators to ground all 50 787s in use in mid-January.

In a fresh sign of the complexity of the international investigation of the batteries, NTSB chief Deborah Hersman said that the agency would hold a forum and a hearing in April.

The forum in mid-April will focus on lithium-ion battery technology and transportation safety.

An investigative hearing later in April will cover the battery system's design and certification by the Federal Aviation Administration (FAA), a certification process that Hersman has criticised.

"The information developed through the upcoming forum and the hearing will help the NTSB and the entire transportation community better understand the risks and benefits associated with lithium batteries, and illuminate how manufacturers and regulators evaluate the safety of new technology," Hersman said in a statement.

Both hearings will be webcast live and open to the public.