SEOUL, South Korea (AP) — South Korean authorities ordered civilians to evacuate on Tuesday after a suspected white phosphorus leak at a U.S. military base near the capital, Seoul.

Officials in the city of Pyeongtaek urged nearby residents to move to a safe location and avoid skin exposure after the leak at the Osan Air Base was detected at around 5 p.m. The alert was lifted several hours later following the completion of decontamination work, according to text messages sent out by the city.

The U.S. 51st Fighter Wing said it established a 300-meter cordon around the base “out of an abundance of caution” after experts conducting a routine inspection identified white phosphorus crystallization that left residue inside a munitions crate.

An explosive ordnance disposal unit was deployed and response plans were immediately activated to ensure the munitions remained fully contained, it said in a statement.

The cordon was lifted after emergency response personnel mitigated the situation, allowing the base to return to normal operations, it said. There were “no injuries resulting from this situation and no risk to personnel on or off installation.”

“The safety of our service members, their families, and our Pyeongtaek neighbors remains our absolute highest priority,” said base commander Col. Ryan Ley.

White phosphorus is a controversial chemical substance used in incendiary munitions and smoke shells. When ignited, it burns at extremely high temperatures and can set buildings ablaze and burn human flesh to the bone.