At least 25 people were killed Wednesday when a plane crash-landed shortly after taking off from South Sudan's capital Juba, an AFP reporter said.

Police were pulling the bodies of men, women and children out of the wreckage of the cargo plane, which crashed into a small farming community on a small island in the White Nile river, close to Juba airport, the reporter said, who counted at least 25 dead.

"Cargo plane heading to Paloch in Upper Nile State crashed just 800 metres from Juba International Airport runway," reported Radio Miraya, a UN-backed station.

The radio said as many as 40 people were feared dead, adding that airport officials had told them only three passengers had survived.

The main fuselage of the plane had ploughed into thick woodland, with debris scattered around the riverbank in a wide area.

Several small farming communities live on the island, and it was not clear if some of the victims were people who were on the ground when the plane hit.

Media reports said it was an Antonov transport plane, but there was no immediate official confirmation. Cargo planes to remote parts of South Sudan often carry passengers too.

Radmir Gainanov, spokesman for Russia's diplomatic mission in Uganda, which also oversees South Sudan, said the embassy was in touch with local authorities, including the defence ministry.

"We are clarifying details," he told AFP from Uganda.

Juba's airport is the busiest in the war-torn country, which is the size of Spain and Portugal combined but with few tarred roads.

The airport hosts regular commercial flights, as well as a constant string of military aircraft and cargo planes delivering aid to remote regions cut off by road.

Civil war broke out in December 2013 when President Salva Kiir accused his former deputy Riek Machar of planning a coup, setting off a cycle of retaliatory killings that has split the poverty-stricken, landlocked country along ethnic lines.

Fighting continues despite an August peace deal, but battles today are far from the capital.