Mozambique's government has refused to let mining giant Rio Tinto use the Zambezi River to transport coal to the Indian Ocean for export, the deputy mining minister said on Friday.

The southern African country's environment ministry worried the operation might harm the ecosystem, Deputy Minister Abdul Razak told AFP.

"For now, according to the information passed on by Micoa (the environment ministry), it is not possible to use the river to export coal," said Razak.

"It is because of the ecosystem. More studies need to be done."

Environmental groups had raised concern about using the river for barge transportation.

The Zambezi flows downward from the Cahora Bassa dam and into the ocean through the Zambezi Delta wetland system, rich with wildlife and birds including some endangered species like the wattled crane.

Infrastructure is the biggest challenge in exploiting the country's massive coal reserves, which have gone largely untapped since independence from Portugal in 1975 due to a civil war that lasted until 1992.

Razak said the Sena railway line to Beira in the centre of the country and the Nacala line in the north were the preferred export routes for coal, which is tucked into the northwestern province of Tete.

But he acknowledged the railroads would not be enough once the mines hit full capacity.

"For now the lines are sufficient for initial coal exports, but we are working with companies eventually to find supplementary means," he said.

Brazilian miner Vale opened a coal mine last year, and has rights to the bulk of the Sena line's capacity. It also owns the controlling share in the Nacala line.

Coal exports are expected to rise 22 percent this year to around $2.3 billion (1.7 billion euros).