South Korean authorities on Tuesday ordered residents to evacuate for several hours after a suspected leak of white phosphorus at the U.S. Osan Air Base, located near Seoul, the Associated Press reports.

The administration of Pyeongtaek urged residents of nearby areas to leave the danger zone and avoid skin contact with the substance.

The leak was detected at around 5:00 p.m. local time. Several hours later, the evacuation order was lifted after decontamination work in the area was completed.

According to local media, police cordoned off an area within a 300–500 meter radius around the base, restricting pedestrian and vehicle traffic. No injuries or serious incidents were reported.

A statement from the U.S. 51st Fighter Wing does not confirm that a white phosphorus leak occurred. The U.S. military said only that, due to a “ground incident,” a safety perimeter with a radius of about 300 meters had been established around the base as a precaution. They also denied reports of a full evacuation of the air base.

The military emphasized that it is taking all necessary measures to ensure the safety of service members, their families, and residents of Pyeongtaek. U.S. Forces Korea provided no additional comment.

White phosphorus is a chemical substance used in incendiary munitions and smoke shells.

When ignited, it burns at an extremely high temperature, can cause large-scale fires, and inflict severe burns, including damage to tissue down to the bone.