The BBC has seen evidence suggesting that the Ukrainian ship being held by pirates off Somalia is carrying weapons and tanks destined for South Sudan.
A copy of the freight manifest appears to show contracts were made by Kenya on behalf of South Sudan's government.
Kenya has repeatedly said the weapons on board the MV Faina are for its army. A South Sudanese official said South Sudan had nothing to do with the tanks.
The MV Faina is currently surrounded by warships monitoring the situation.
Last week, the Somali government said the ship's owners were involved in direct negotiations with the pirates, who are demanding a $20m (£11m) ransom.
A copy of the MV Faina's manifest given to the BBC appears to confirm that the contract was issued on behalf of South Sudan, although the Kenyan defence ministry is named as the consignee.
Contract numbers for tanks, rocket-propelled grenade launchers and anti-aircraft guns contain the initials GOSS, which military sources tell the BBC is a reference to the Government Of South Sudan.
This is an acronym commonly used in Sudan.
But Francis Nazario, head of South Sudan's mission in Brussels, said he had seen the manifest and it did not prove anything.
"What I know is that we have nothing at all to do with the content of this ship, and the ship was not heading for South Sudan," he told the BBC's Focus on Africa programme.
"I think if there was anything like that we would not hide it because constitutionally we have the right to do so, to bring arms from anywhere."
The Kenyan government has not yet commented on the document but it has been presented to the defence and foreign relations committee of Kenya's parliament.
Kenya has repeatedly insisted that the shipment was part of a programme to restock its military.
The BBC's Karen Allen in Nairobi says that this will be a huge embarrassment to the Kenyan government.
Although the import of military hardware is not illegal, it does put Kenya in a tight spot diplomatically, our correspondent says, not least because it was Kenya which helped broker an end to the civil war between South Sudan and the government in Khartoum in 2005.
Meanwhile, a Kenyan court has granted bail to Andrew Mwangura, a spokesman for the Kenyan chapter of the Seafarers Assistance Programme, who had been arrested after he said the tanks were bound for South Sudan.
Mr Mwangura was charged with making alarming statements and illegal possession of marijuana.
However, he has not yet raised the 200,000 shillings, ($2,700) and so he is still in custody.
The MV Faina is currently moored off the coast of Somalia, close to the town of Hobyo. There have been conflicting reports about where its cargo was destined for since it was captured two weeks ago.
Last week, Western military experts told the BBC that the tanks on board the MV Faina were going to Sudan and that the shipment indicated an arms race between North and South Sudan had begun.
They are reported to both be building up their forces ahead of a referendum on independence for the South in 2011.
The pirates want a $20m ransom for the MV Faina and its valuable cargo
The military experts, who spoke on condition of anonymity, said a previous delivery of tanks had taken place last November.
Helmoed Heitman, Africa correspondent for Jane's Defence Weekly, also said he had reports that more than 100 T-72 and T-55 Russian tanks have been received by the southern Sudanese in recent months.
"If these reports are true, they could change the regional military balance," he told the BBC.
"Kenya could be seen as playing the same role as Cuba did during the Angolan civil war - when they armed the MPLA."
The experts said the tanks would most likely be dug in along Sudan's north-south border, with the tanks using their guns to protect military installations.