Taiwan's last major rail crash was in October 2018, when an express train derailed while rounding a tight corner on the north-east coast, killing at least 18 people and injuring almost 200.
A further 70 people were still thought to be trapped in carriages inside the tunnel while around 60 were rushed to hospital.
Authorities said it apparently hit a truck that slid off a road leading to a nearby construction site.
"This is our initial understanding and we are clarifying the cause of the incident", he added.
Belize's Ministry of Foreign Affairs took to its official Twitter account and said, "Belize extends honest condolences to the Government and People of Taiwan following the sad news of a train accident in eastern Taiwan".
The accident occurred on Taiwan's eastern railway line around 9:30 a.m. (0130 GMT) near the coastal city of Hualien.
Pictures published by local newspaper UDN showed the front of the train inside the tunnel had been pulverised into a twisted mesh of metal.
Photos and video taken after the crash showed a scene of cars torn apart inside the tunnel and passengers crawling out of the wreckage.
"Many people were crushed under train seats in the collision".
Taiwan's Central Emergency Operation centre says rescuers are struggling to access four carriages inside the tunnel that are "badly damaged".
"It felt like there was a sudden violent jolt and I found myself falling to the floor", an unidentified female survivor told the network.
The train, travelling to Taitung, came off the rails in a tunnel just north of Hualien causing some carriages to hit the wall of the tunnel, the fire department said in a statement.
The accident came at the beginning of a long weekend for the traditional Tomb Sweeping Day holiday.
The train had only partially emerged from a tunnel, and with much of it still inside, many escaping passengers were forced to scramble out of doors and windows and scale the sides of the train to walk along the roof to safety.
During the festival, people return to ancestral villages to tidy up the graves of their relatives and make offerings.
Taiwan's mountainous east coast is a popular tourist destination.
With the help of multiple tunnels and bridges, it winds its way through towering mountains and dramatic gorges before entering the picturesque Huadong Valley.
"If there is a request from the Taiwan side regarding support, we want to consider possible assistance", he said.
The last major train derailment in Taiwan was in 2018, which left 18 people dead.
The National Fire Service confirmed that 48 people were dead and more than 100 injured.
Officials say the train collided with a construction truck that had rolled down a slope onto the rails.
In 1981, 30 were killed in a collision in its north, while a train crash in 1991 killed 30.