Ivory from southern Sudan, the Democratic Republic of Congo and the Central African Republic makes its way to dealers in Khartoum and Cairo, said Martin, who is based in Kenya.
Martin said a 20-year civil war in southern Sudan had made it difficult to determine exact elephant numbers, but figures available indicate that the numbers fell to 40,000 in 1992 from 133,000 in 1976.
"Every trader we talked to said the Sudanese national army has been doing the killing, they possess the necessary firearms and ammunition. They also have access to government transport to move tusks to Khartoum and Omdurman," Martin told a news conference in Nairobi.
A Sudan armed forces spokesman declined to comment on the reports.
Martin said three quarters of the illegal ivory was made into chopsticks and other items and sold to Chinese nationals, thousands of whom have moved into Sudan to work in the petroleum, construction and mining sectors.
China has been the largest importer of illegal tusks since the mid 1990's, despite a ban on ivory trade by the Convention on International Trade in Endangered Species (CITES) in 1990.
"Every trader we talked to said the Sudanese national army has been doing the killing"
Conservationist Esmond Martin
"The driving force behind these elephant killings especially in the recent past is directly linked to the activities of the Chinese. Not restricted to Chinese expatriates in Sudan they extend to traders in China who buy these tusks and encourage the trade," he said.
Martin said prices for ivory in Khartoum and Omdurman, a city across the River Nile from Khartoum, were between $44-$148 per kg compared to $15.50-$43.60 per kg in 1997 due to an increase in demand.
Sale of ivory items in Sudan is legal provided the shopkeeper has a government licence and the items are carved from ivory obtained before the CITES ban, but Martin said he counted over 11,000 items made from post-CITES ivory in souvenir shops in Khartoum.
Ivory items such as walking sticks and daggers are also in demand in South Korea, Saudi Arabia and the United Arab Emirates, he said.