Taiwan's top military official was among eight people killed on Thursday after the helicopter carrying them made a forced landing in a mountainous area near the capital Taipei, the defence ministry said.
The main portion of the helicopter lay in a northern forest wreathed in mist, with its blades shattered into pieces, as dozens of rescuers combed the wreck for survivors, pictures released by emergency authorities showed.
The chief of general staff, Air Force General Shen Yi-ming, died in the incident, while five of the 13 people aboard survived, the military said in a statement.
"Eight of our colleagues were killed," a military spokesman told a news conference broadcast live on television.
The defence ministry said it had dispatched a rescue team following the Black Hawk helicopter's forced landing in New Taipei City, after aviation authorities lost contact with the craft at 8:22 a.m.
The helicopter had left Taipei on a mission to visit soldiers in the northeast county of Yilan ahead of Lunar New Year at the month-end.
The incident comes a week before democratic Taiwan holds presidential and parliamentary elections on January 11.
President Tsai Ing-wen, who is seeking re-election, cancelled all campaign activities until Saturday, and urged authorities to make every effort at rescue. She is scheduled to deliver a speech at 3 p.m.
The United States, which has no formal diplomatic ties with Taiwan, but is its strongest international backer and main arms supplier, sold the island 60 UH-60M Black Hawk helicopters in 2010.
It was not immediately clear if the helicopter in Thursday's incident was one of them, however.
The incident was the latest in a series of aviation accidents in Taiwan, after the 2018 crash of a Black Hawk helicopter off its east coast killed six people aboard and the crash of an F-16 fighter jet killed a pilot the same year.
In 2016, the navy fired a supersonic missile in error, hitting a fishing boat in waters that separate Taiwan from diplomatic rival China.
China, which claims Taiwan as its territory to be brought under Beijing's control by force if necessary, regularly calls the island the most sensitive issue in its ties with the United States.
Taiwan says it is an independent country called the Republic of China, its official name.