The chief of the general staff, Shen Yi-ming, was among eight senior officers -- including three major-generals -- who died when their Black Hawk helicopter smashed into mountains near Taipei.

The 62-year-old general and his entourage were on a routine mission to visit soldiers in northeast Yilan county for the upcoming Lunar New Year when the incident happened.

Flags at all military units will fly at half-mast for three days as Shen was the highest-ranking military official to die while on official duty, the government said.

Lieutenant-general Tsao Ching-ping, one of five survivors, told rescuers in footage broadcast on local TV: "I am okay... two others are injured and only I can walk."

"There is one more person who's more seriously wounded and two or three people in the cabin... two more with no signs of life."

President Tsai Ing-wen's office said she would cancel all campaign activities for three days after the tragedy. The ruling Democratic Progressive Party will also suspend campaigning for three days.

Tsai is seeking a second term against Kaohsiung city mayor Han Kuo-yu of the Kuomintang (KMT) party in the January 11 elections when Taiwan will also elect a new parliament.

- 'Deeply saddened' -

Han and the KMT also offered condolences to the victims and announced that they will stop campaigning for two days.

"Today is a day that we are deeply saddened because several of our most distinguished generals died while on official duty," Tsai said at a briefing for the incident.

"I've asked the defence minister to maintain stable military morale at this time to ensure steady military and defence operations for the safety and stability of our country."

There have been a number of incidents involving Black Hawk helicopters -- purchased from the United States -- in recent years in Taiwan.

In 2018 a chopper belonging to a government rescue agency crashed during a medical mission off outlying Orchid Island, killing six people on board in an incident attributed to human error.

There were also two crash landings in 2016 and 2018 with no casualties.

The US has remained Taipei's most powerful unofficial ally and its leading arms supplier despite switching diplomatic recognition to Beijing in 1979.

"We hope that our steadfast commitment to supporting Taiwan's security will honour their memory," Washington's de facto embassy, the American Institute in Taiwan (AIT), said in a statement.

"I was privileged to work closely with General Shen in our joint efforts to strengthen the US-Taiwan security relationship. With his keen insight and good humour, he was a valued leader, colleague and friend. He will be sorely missed," said AIT director Brent Christensen.

US Joint Chiefs Chairman General Mark Milley described Shen as "an exceptional leader to his people and a champion for Taiwan's defense and regional security."

"We are grateful for the service he rendered so selflessly and cherish our friendship and strong defense relations with Taiwan," he said in a statement.

The UH-60M helicopter carrying 13 people disappeared from radar less than 15 minutes after taking off, said Air Force Commander Hsiung Hou-chi, adding that the ministry had set up a taskforce to investigate the incident.

"We are investigating whether (the cause) was environmental or mechanical," he told reporters.

The ministry has dispatched ground troops and rescue helicopters to the crash site in northeastern Taiwan. It said survivors will be carried off the mountains for treatment rather than being air-lifted due to bad weather.