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By Ellie Cook
Senior Defense Reporter
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A white phosphorus leak at a joint United States-South Korean military base triggered a temporary evacuation order for local residents near the facility south of Seoul on Tuesday, officials said.
The incident occurred at Osan Air Base, Seoul's safety ministry said. The site is home to the U.S. Air Force's 51st Fighter Wing, which said it had briefly put a 1,000-foot cordon in place shortly after 5 p.m. local time after experts found a "white phosphorus crystallization" and residue inside a crate of unspecified munitions.
The air base was not fully evacuated, the U.S. military said, adding the cordon was established "out of an abundance of caution," and that it was an "isolated situation." It was later lifted and no injuries were reported.
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Officials in Pyeongtaek, the nearest city to the air base, said the evacuation order was pulled back at 6:13 p.m. local time.
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White phosphorus is a waxy substance that ignites when exposed to oxygen, producing toxic fumes. It's often used by military forces, including American troops, in grenades or artillery shells, to light up battlefields or create smoke screens during battles. It's not considered a chemical weapon under the Chemical Weapons Convention (CWC), but its deliberate use against civilians as an incendiary weapon is banned.
Roughly 28,500 U.S. troops are stationed in South Korea, and the U.S. has had a continuous military footprint in the country since the end of the Korean War in 1953.
South Korea and North Korea remain technically at war after an armistice agreement stopped the conflict, but the decades since have been marked by fluctuating relations across the Demilitarized Zone separating the two Koreas.
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South Korea leans heavily on the U.S. for military support in the face of North Korea's burgeoning nuclear arsenal, intercontinental ballistic missile tests and defense investment. Seoul relies on the U.S. to come to its defense in the event of a North Korean attack, including with America's vast nuclear weapons stockpile.
Pyongyang, meanwhile, is sharply critical of U.S. ties with Seoul and frequent joint exercises.
U.S. President Donald Trump suggested last year that South Korea should be paying more for a U.S. military presence on the divided peninsula, although the U.S. has not confirmed whether it will move any of its current forces permanently away from South Korean soil.
Update 7/28/2026 at 11:40 a.m. ET: This article was updated with additional information.
Contact Newsweek editors on this story: Tobias Meyjes, Tony Phillips and Sam Wilson.
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