At least 25 people, many of them children and teenagers, have been killed after a bus carrying students plunged into a deep ravine on Cape Verde's Fogo Island, in one of the deadliest road accidents in the country's recent history.

The accident occurred late on Saturday on a mountainous road between Chã das Caldeiras and Campanas de Cima in northern Fogo. The bus veered off the road and plunged approximately 30 metres down a steep slope.

Authorities said 18 people were injured in the crash, with four in serious condition. Several of the injured were taken to São Francisco de Assis Regional Hospital on Fogo for treatment. Some passengers were trapped inside the wreckage, while the rocky terrain and difficult access complicated rescue operations, which continued through the night.

Many of those travelling on the bus were primary and secondary school students from the Curral Grande and Ponta Verde areas.

Education authorities have clarified that the excursion was not organised by local schools. Instead, it was an annual trip privately arranged by the bus driver for students he regularly transported during the academic year.

The driver, who had reportedly organised the excursion as a gift for the pupils, was among those killed.

The group had been returning from an excursion to Chã das Caldeiras, a village located inside the crater of Pico do Fogo, Cape Verde's highest mountain and an active volcano, when the accident occurred.

Authorities have not yet established what caused the driver to lose control of the vehicle. An investigation into the crash is underway.

The scale of the accident has shocked Cape Verde, an Atlantic archipelago of around half a million people located off the western coast of Africa.

President José Maria Neves described the accident as a “great catastrophe” and offered condolences to the families of those killed while wishing the injured a rapid recovery. The president, prime minister and health minister were expected to travel to Fogo to meet victims' families and assess the response to the disaster.

The government has declared two days of national mourning. Authorities said the loss of so many lives represented an “irreparable loss” for the country and Cape Verdean society. Local authorities in São Filipe also declared two days of municipal mourning and convened an extraordinary meeting to determine what assistance should be provided to affected families.

Fogo is one of Cape Verde's most mountainous islands, dominated by the nearly 2,830-metre Pico do Fogo volcano. Roads through the island's interior pass through steep volcanic terrain, including the area where Saturday's crash occurred.

Officials have cautioned against drawing conclusions about the cause before the investigation is completed. Road accidents on this scale are relatively rare in Cape Verde, where roads are generally considered to be in good condition.

For the communities of Curral Grande, Ponta Verde and the wider island of Fogo, however, the disaster has been particularly devastating because so many of the victims were young students returning home from what had been intended as an annual excursion.