As many as 200 North Korean workers may have been killed in a tunnel collapse at the country's nuclear test site, according to a Japanese news report Tuesday.

Around 100 people were trapped when an unfinished tunnel at the Punggye-ri site caved in, and an additional 100 people could have died in a second collapse during a rescue attempt, Japanese television station TV Asahi reported.

TV Asahi reported that North Korea's Sept. 3 hydrogen bomb test at the site, its sixth nuclear test, could have weakened the area and caused the tunnel to collapse. It's unclear exactly when the tunnel collapsed.

No officials have confirmed the Japanese station's report.

North Korea said at the time that the test - a miniaturized hydrogen bomb capable of fitting on an intercontinental ballistic missile - was a "perfect success" and a "meaningful" step forward.

The U.S. Geological Survey, meanwhile, said it recorded a 6.3-magnitude earthquake near the North Korean nuclear test site in the northeast.

Scientists later warned that the nearby mountain, Mantap, had "tired mountain syndrome" and could collapse and release radiation, The Washington Post reported.

News of the collapse comes days before President Trump is set to leave for Asia, his first trip there as president.

Trump last week raised the possibility of a potential presidential visit to the Demilitarized Zone (DMZ) between North and South Korea, but a senior administration official said Tuesday that Trump would not visit the DMZ on his 12-day trip to Asia that starts Friday.