Nairobi - Two men on Monday pleaded guilty to illegal ivory possession in Kenya after they being arrested in one of East Africa's biggest seizures in recent years, the Kenya Wildlife Service (KWS) said.

Paul Udoto, spokesman for KWS, said the two men - a Kenyan and a Tanzanian - were arrested with 512 kilograms of elephant tusks near the Kenyan-Tanzania border on Saturday.

"We are estimating this is from 35-40 elephants," he told the German Press Agency dpa. "I can tell you that is an incalculable loss to our biodiversity. It is incredible that a person would be so cruel as to do that."

Udoto said that investigations were ongoing to find out where the elephants were killed.

The two men are due to be sentenced on May 4.

The ivory trade has been banned since 1989, but illegal sales have thrived with demand being fuelled largely by China and Japan.

A major operation across five African countries late last year, conducted with the cooperation of Interpol, saw dozens of suspected ivory dealers arrested and one tonne of ivory products seized.

Hundreds of kilogrammes of ivory destined for the Asian markets were also seized at Nairobi's Jomo Kenyatta Airport last year.

Japanese and Chinese traders in November took part in a controversial ivory sale in South Africa, which raised raised 6.7 million dollars.

The UN-backed Convention on International Trade in Endangered Species (CITES) gave the go-ahead for the governments of South Africa, Namibia, Zimbabwe and Botswana to sell ivory accumulated in their national parks in one-day sales.

However, animal rights groups objected to the auctions, saying all sales of ivory - even legal - stimulate black market trade in the so- called "white gold" and, consequentially, elephant poaching.

Kenya's elephant population has almost doubled to 27,000 since CITES banned the ivory trade.

However, the herds have yet to fully recover from the widespread poaching that threatened Kenyan elephants with extinction.

There were an estimated 167,000 elephants in Kenya in 1973. (dpa)