The lone adult survivor of a South Sudan plane crash said Saturday that he cradled a stranger's baby in his arms as doomed passengers shouted the aircraft was going down, and he and the child were the only people who lived through the carnage.

Wuor Arop also told The Associated Press from his hospital bed where he was recovering with limbs broken in six places and a head injury that the plane carried more than 30 unauthorized passengers.

The Soviet-built Antonov An-12 was had taken off from South Sudan's capital, Juba, on Thursday and was headed for the Paloich oil fields with a crew of six when it crashed, according to South Sudan's Civil Aviation Authority. Thirty-seven people died in the crash, said officials.

The only other survivor is a 13-month old baby not related to Arop whom he cradled in his arms during the crash. She survived with a broken leg and a cut on her forehead.

Shortly after takeoff other passengers started shouting that the plane was crashing, said Arop.

"The baby I grabbed, [she] was near me," he said. "Plus my friend who was near me, he stepped on me so I grabbed him."

He said he remembers his friend calling out his name just before impact, his last memory before waking up in hospital.

Arop said he paid 500 South Sudanese pounds, less than $30 in Juba street exchange rates, to an unofficial "dealmaker" for a seat on the cargo flight. He said no ticket was issued and the fee was split between the pilot and the broker.

He was one of nine people besides the crew who had seats on the plane, while the other passengers sat among the cargo, which included cheap sandals, crackers, cans of beer, and medicine, he said.

The plane's three black boxes were recovered, Minister of Transportation Kuong Danhier Gatluak said Saturday. The government has banned all AN-12s from flying in South Sudan for the time being, he said.

Juba, South Sudan plane crash

At least 37 people were killed when the plane crashed over Juba, South Sudan. (Google)

William Deng of Allied Services Ltd., the company that operated the Tajikistan-registered plane, declined to answer questions when reached on the telephone Saturday.

Arop said he was flying to Paloich to return to his job with the Danish Refugee Council after visiting his family in Juba.

There are no commercial passenger flights to Paloich and roads are impassable due to South Sudan's civil war and rainy season. The UN Humanitarian Air Service, which transports aid workers around the mostly road-less nation, also flies to Paloich.

South Sudan's presidential spokesperson Ateny Wek Ateny previously said the flight was chartered by a local businessman, but gave no more details.

The flight was carrying its maximum cargo of 15.5 tonnes when it took off, according to Civil Aviation Authority head Stephen Warikozi, citing the cargo manifest. Warikozi said they have not found a passenger manifest.