South Korea is hosting the anti-missile defence system in the Seongju region, about 250 km (155 miles) from the North Korean border as part of efforts to counter a growing missile threat from the North.
The Baengnyeong drone was found days after another one was spotted in Paju, a city that sits near the demilitarized zone which divides the two countries.
Though he often denounced China on the campaign trail, Trump is now leaning on Beijing to help rein in its ally North Korea's weapons program, a move that has sparked anguish among Asian allies who fret China might exact a quid pro quo to make Washington go easy on other regional issues.
"Around 10 photos were found in the Japan-made Sony camera attached to the drone", said a South Korean military official Monday, who embargoed the story until Tuesday. Further analysis was underway. In 2014, several other suspected North Korean drones were found south of the border and experts said they were crude and low-tech but could be considered as a potential security threat.
Drones are a relatively new addition to North Korea's arsenal. "The drone was equipped with a twin-engine and Global Positioning System device".
In one of its latest vitriolic backlashes, the state-run newspaper Rodong Sinmun ran a statement from the so-called Pan-Korean Emergency Measure Committee for Opposing Nuclear War Drills Against the DPRK Monday, saying the antimissile system evades "rebukes of neighboring counties and curry favor with the US administration to prolong their remaining days".
Mattis called North Korea the "most urgent and unsafe threat" to peace and security, and said its continued pursuit of nuclear weapons and the means to deliver them has increased in pace and scope.
The article continued it was "another heavy crime and unpardonable pro-U.S. act of treachery".
The current tensions in the Korean peninsula date back to the Cold War, when the North was controlled by a communist regime under the influence of China and the South by the United States. She is on the honorary board of the International Paralympic Committee.
North Korea's state-run Korean Central TV Broadcasting Station aired two satellite images of the THAAD deployment site and the ballistic missile defense batteries in May.
Some devices are imported from a Chinese aviation technology company, perhaps through a front company in Hong Kong, Bermudez said.