Nearly 1,000 hooded vultures have died in a mass poisoning in Guinea-Bissau, pushing the endangered species towards the brink of extinction in Africa, according to conservationists.

Vultures were seen apparently searching for water and “bubbling from their beaks”, and hundreds were found dead on the outskirts of two towns, Bafatá and Gabú, which are 30 miles apart, over the past two weeks.

The poisoning is the biggest mass death of vultures for more than a decade, according to the Vulture Conservation Foundation.

The likeliest cause of death is accidental poisoning after strychnine – which is banned in Europe – was used to control the feral dog population around rubbish dumps where vultures, which scavenge on dead animals, also feed.

Guinea-Bissau is in political turmoil, its institutions are paralysed and travel is difficult, so only one vulture carcass has so far been retrieved for a postmortem to determine its cause of death.

Vulture populations have plummeted in Africa in recent years. The birds are often the accidental victims when poisoned baits are used to kill lions, hyenas and other wildlife.

Vultures are also killed in “sentinel poisoning” incidents in which poachers deliberately target the birds because their rapid arrival above animal carcasses can help police and wildlife rangers identify elephant and rhino poaching.

In Africa, mass poisonings largely linked to wildlife crimes are causing the loss of thousands of vultures each year. Photograph: Andre Botha/Vulture Conservation Foundation

“The poisoning across Africa has pushed this species to the verge of extinction,” said José Τavares, the director of the Vulture Conservation Foundation. “The hooded vulture is critically endangered at a global level and Guinea-Bissau has one of the best populations in Africa. This mass poisoning will have a significant impact on the species.”

Vulture populations in India have declined by 99% after the birds ingested a toxic veterinary drug, diclofenac, through cattle carcasses it fed on.

In Africa, mass poisoning events largely linked to wildlife crimes are causing the loss of thousands of vultures each year, with other single incidents killing 600 vultures and 400 vultures in southern and eastern parts of the continent.

Elephant and other wildlife poaching is not widespread in Guinea-Bissau but Tavares said the precise cause of this poisoning would remain unclear until the authorities in Guinea-Bissau could collect more carcasses for toxicology tests. All but one of the dead birds have been incinerated so far to prevent any wider risk to public health.

The Vulture Conservation Foundation is working with agencies in Guinea-Bissau in an attempt to collect and freeze more carcasses, which can then be flown overseas for testing.

While vulture species continue to disappear in Africa and Asia, three of the four species found in Europe are increasing. According to Tavares, vulture populations have been revived with reintroduction programmes and effective EU wildlife regulations, including allowing farmers in regions to leave out parts of dead livestock for the vultures to feed on.