By Martin Plaut
BBC News, Africa analyst
Southern Sudanese are said to be seeking supplies of heavy weapons
Military experts say the cargo of tanks captured by Somali pirates off the Horn of Africa was destined for South Sudan.
They believe the shipment is an indication that an arms race between the government in Khartoum and South Sudan is under way.
Experts say the south may have 100 Russian-built tanks in its arsenal.
Both northern and southern Sudan are reported to be building up their forces ahead of the possible independence of the south in 2011.
The seizure of the Ukrainian vessel off the coast of Somalia has lifted the lid on what experts say is an arms build-up in Sudan.
Western sources, who spoke on condition of anonymity, told the BBC that the tanks on board the ship were going to southern Sudan, despite denials from Kenya and from Ukraine.
The experts say that there has been more than one shipment of tanks - a previous delivery took place in November last year.
Helmoed Heitman, Africa correspondent for Jane's Defence Weekly, says he has reports that more than 100 T-72 and T-55 Russian tanks have been received by the southern Sudanese.
"If these reports are true, they could change the regional military balance," he said.
"Kenya could be seen as playing the same role as Cuba did during the Angolan civil war - when they armed the MPLA."
Tanks are notoriously difficult to operate and require frequent maintenance.
The Western sources who spoke to the BBC suggest their most likely use would be dug in along Sudan's north-south border, with the tanks using their guns to protect military installations.
The south is not alone in seeking fresh weaponry, with the authorities in Khartoum reported to be re-equipping their armed forces from Malaysia, China and North Korea.
As a result the peace agreement signed between the Sudanese government and the southern Sudanese rebel movement in 2005 - ending one of Africa's longest and bloodiest conflicts - is looking increasingly frayed.
It halted a civil war that lasted decades and cost two million lives.
But Sudan's oil wealth, and the possibility that southern Sudan will secede after a referendum in 2011, appears to be stoking a new arms race.