U.S. air-safety regulators have taken the unusual step of singling out Samsung Electronics Co.’s Galaxy Note 7 smartphone as a potential airborne fire hazard, urging passengers to avoid using the devices entirely on board airliners, dealing another blow to the technology giant’s smartphone recovery efforts.

In a brief but strongly worded statement released late Thursday, the Federal Aviation Administration said that “in light of recent incidents and concerns” involving the smartphones, the agency “strongly advises passengers...