The US Consumer Product Safety Commission (CPSC) has urged consumers who own Samsung Galaxy Note 7 smartphones to "power them down and stop charging or using" the devices.
The warning on Friday was based on recent reports involving lithium-ion batteries in certain Note 7 devices that have resulted in fires, Xinhua news agency reported.
"These incidents have occurred while charging and during normal use, which has led us to call for consumers to power down their Note7s," it said in a press release.
The US agency said it was working with Samsung to formally announce an official recall of the devices as soon as possible.
"CPSC is working quickly to determine whether a replacement Galaxy Note7 is an acceptable remedy for Samsung or their phone carriers to provide to consumers," it said.
On Thursday, the US Federal Aviation Administration "strongly" advised passengers "not to turn on or charge" Note 7 devices on board aircraft and "not to stow them in any checked baggage".