The Antonov An-12 plane was heading to Paloch, Upper Nile State, and crashed 800m (half a mile) from the runway.
In a statement, Ukraine-based Antonov said the plane had not been airworthy.
It said the plane, which was built in 1971, "was is no state to fly because it failed to undergo timely technical servicing... that should have included work on extending its resources and exploitation timeframe", AFP news agency reported.
South Sudan authorities warned that the death toll could rise as the debris was cleared.
The plane crashed into a farming community on an island on the White Nile River but so far all the victims recovered were from the aircraft, the Red Cross said.
Presidential spokesman Ateny Wek Ateny said all of the plane`s passengers were from South Sudan.
He said five of the six crew members were Armenian, while the sixth member was Russian. Armenia`s foreign ministry has confirmed that five of its nationals were killed.
The head of the Civil Aviation at Juba airport said emergency officials had secured the site of the crash and were "in the stage of recovering bodies and black box [flight recorder]".
While the cause of the crash is still unclear, the presidential spokesman told a news conference that it might have been down to engine failure.
Witnesses said bodies, debris and cargo were strewn over a wide area along the river bank.
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