Zimbabwe's rhinos are being wiped out amid a surge in poaching driven by Chinese demand for the animals' horns, a wildlife conservation group warned today.

Around 120 rhinos have been killed since last March to feed the lucrative Chinese black market, said Johnny Rodrigues, chairman of the independent Zimbabwe Conservation Task Force.

Since last year economic collapse and the breakdown of law and order have contributed to a rapid escalation in poaching by organised gangs. "In the past 15 months we've lost 120 rhinos, and we're still losing two to four per month," Rodrigues said. "We used to have 1,000 in this country."

The exact size of Zimbabwe's current rhino population is debated. Save the Rhino, a British-based charity, puts the total at above 700. Rodrigues says it is about 400. Both agree the situation represents a crisis.

Rodrigues said that Zimbabwe's trade links with China, where the rhino horn is highly prized as medicinal, are a driving factor. "We're now down to about 400 rhinos, black and white, since the opening of the Chinese market. Normally the first thing the Chinese ask when they come here is, 'Have you got rhino? Have you got rhino?'"

He added: "It's all linked to the top. All those corrupt ministers are trying to cream off as much as possible before the next election. But if the carnage continues over the next two years we'll have nothing left. The devastation taking place is not sustainable."

A rhino horn can sell for thousands of pounds on the black market. Along with Chinese medicine, the horns are used for ornamental dagger handles in some Middle Eastern countries.

Rodrigues said gangs were now using a Chinese-made version of a tranquillising agent that can be fired noiselessly from a dartgun to avoid drawing attention. The gangs then chop off the horn and leave the unconscious animal for dead. "They don't reverse the tranquilliser, so the rhino overheats and dies," Rodrigues said. "Anyone who then finds it can't eat the meat – you will die if you do.

"The removal of the horn is very harsh. They use an axe and disfigure the rhino's face. The humane thing to do is put a bullet through its head and burn the carcass."

Rodrigues is preparing to hand a dossier to the prime minister, Morgan Tsvangirai, in the hope that the country's unity government will take tougher action.

Government vets have made attempts to de-horn rhinos so they no longer have value for poachers, but the process must be repeated because the horns regrow. The army and police have been called in to conservation areas and national parks to defend the animals, but it is alleged that some soldiers turn poachers themselves.

Poachers have little to fear. Even those who are caught are usually freed on minimum bail because there is often no fuel to bring them to court.

Zimbabwe's Parks and Wildlife Management Authority challenged Rodrigues' claims, but refused to give a figure for the rhino population.

"We definitely have more than 400," said Vitalis Chadenga, director of conservation. "But it's true we're facing an upsurge in the poaching of rhinos. This has taken place mostly on private farms, though parks have also suffered losses.

He insisted: "The government takes it very seriously. We have de-horned some rhinos and relocated some to safer areas where we can afford them maximum protection … if you come here in 10 years' time you will still see the rhino. They are safe but they are under threat. There is not a soft touch in terms of law enforcement."

The Convention on International Trade on Endangered Species has said it will discuss the threat to Zimbabwe's rhinos at its next meeting in July.

The government has said tourism is one of its best opportunities for quick economic revival. But Rodrigues warned: "We were the jewel of Africa, but we've gone back 15 or 20 years. The wildlife has been decimated to such a stage that there'll be nothing left for tourists when they come back to the country."