Three soldiers were killed when a tank slipped and plunged into a river during heavy rains following an annual firing drill in southern Taiwan, officials said Tuesday.

The CM11 armoured vehicle carrying five soldiers was returning to camp in southern Pingtung county around 10:30 am (0230 GMT) after completing the firing test when it slipped from a bridge and fell upside down into the Wangsha river, the army said.

The driver managed to escape with light injuries but four others were trapped inside the vehicle and showed no signs of life when they were rescued.

The army initially said they were all killed but later revised the death toll to three as one soldier was revived after emergency treatment and was transferred to a military hospital in neighbouring Kaohsiung city.

"We are still investigating the cause of the accident," said Alfonso Yang, a military spokesman.

According to the army, the driver was unable to make a left turn when the vehicle possibly malfunctioned and then fell into the river.

President Tsai Ing-wen expressed her condolences and demanded the military speedily investigate the cause of the incident, her spokesman said.

The accident happened days before Tsai is due to preside over an annual live-fire exercise codenamed "Han Kuang 32" (Han Glory), also in Pingtung county next week, the island's main yearly drill.