Poachers have killed nearly half of Mozambique's elephants for their ivory in the past five years, the US-based Wildlife Conservation Society said Tuesday.

A Mozambique government-backed survey showed a dramatic 48 percent decline in elephant numbers from just over 20,000 to an estimated 10,300, the WCS said in a statement.

"This decline is due to rampant elephant poaching in the country's most important elephant populations," the statement said.

Remote northern Mozambique, which includes the Niassa National Reserve, was the hardest hit, accounting for 95 percent of elephant deaths, reducing the population from an estimated 15,400 to an estimated 6,100.

The figures can be explained by the arrival of poachers from Tanzania, where the elephant population has already been decimated, according to Alastair Nelson, director of WCS in Mozambique, whose organisation administers the Niassa Reserve.

"The major issue is one of governance. The north has always been a remote and poorly governed area, with an underlying level of corruption," he told AFP.

"Some district police and border guards are being paid off, some even rent out their own firearms."

The aerial survey found that in some parts of the country nearly half the elephants seen were already dead.

Elephant tusks are prized in Asia, where they are carved into ivory statuettes and jewelry.

Across Africa, up to 30,000 elephants are estimated to be killed illegally each year to fuel the ivory trade.

An estimated 470,000 wild elephants remain in Africa, according to a count by the NGO Elephants Without Borders, down from several million a century ago.

Maputo has been slow to start the fight against poaching. Under international pressure, the country adopted in June 2014 a new biodiversity law criminalising the killing of protected animals.

Before that, poachers were simply fined for illegal possession of a weapon.

On May 14, police in Mozambique said they had seized 1.3 tonnes of elephant ivory and rhino horn -- the result of killing about 200 animals -- in the country's biggest-ever find of illegal wildlife products.

An Asian man was arrested on the outskirts of the capital Maputo at a house where the stash was stored.

Rhinos are believed to be extinct in Mozambique, but hunters from the country are often armed by transnational crime syndicates to kill rhinos and elephants across the border in South Africa.

The police raid discovered 340 elephant tusks, weighing 1,160 kilogrammes, and 65 rhino horns, weighing 124 kilogrammes.

The cache was reported to have a street value of about $6.3 million, with rhino horn prized for use in traditional medicine in Asia.