U.S. Defense Secretary Jim Mattis on Wednesday visited a former American air base in southern Vietnam that will soon become the biggest-ever U.S. cleanup site for contamination left by the defoliant Agent Orange during the Vietnam War.

Standing near a skull-and-crossbones warning sign meant to keep people away from toxic soil, Mattis was briefed by Vietnamese officials about the massive contamination area.

In a possible sign of the sensitivity surrounding Agent Orange in Vietnam, where millions of people are still suffering its effects, reporters were not allowed to attend the outdoor briefing for Mattis at Bien Hoa Air Base.