By Amber Henshaw
BBC News, Khartoum
Evidence that a shipment of tanks and military equipment seized by Somali pirates was destined for South Sudan adds weight to suspicions that the former enemies in Sudan's long civil war are once more stockpiling weapons.
Some southerners fear renewed conflict after the 2011 referendum
The BBC has seen a copy of the Ukrainian MV Faina's freight manifest suggesting that the Russian-made hardware was bought by South Sudan's government.
The semi-autonomous south of Sudan is said to be beefing up its military ahead of the 2011 referendum that could see the north and south of the country split.
Experts say a large portion of the south's budget is being spent on military equipment, salaries and training even though vast sums are still needed to rebuild a region devastated by civil war.
War raged for more then two decades until a peace deal was signed in 2005. The north and south are now uneasy, distrustful partners in a coalition government.
For its part, the dominant National Congress Party (NCP) led by President Omar Al-Bashir, is keeping quiet.
The north's military focus is the on-going conflict in Darfur, which started five years ago when mostly non-Arab groups took up arms against the government.
There is a UN arms embargo that covers the use of weapons in the Darfur region. The fear is that if anyone delved too deep the north could be exposed for breaking that embargo, with potentially serious consequences at a delicate time for the NCP.
One political insider in Khartoum said: "The north is being very quiet about the tanks. They can't say anything about this because they are doing the same. If they say anything it will open a can of worms."
The Sudan People's Liberation Movement (SPLM) - the former rebels who now run the south - and the Kenyan government have denied the hi-jacked shipment was bound for South Sudan and the reason for secrecy is clear: this cargo and the previous orders amount to a violation of the peace deal that brought an end to the lengthy civil war.
Under that deal, the south is due to vote in 2011 in a referendum on whether to secede from Sudan.
But many southerners fear that Khartoum could attack if the south votes for independence.
A Sudanese expert with close ties to the SPLM government said the military build-up was to deter conflict, not trigger it: "The south is preparing for the worst but expecting the best.
"It's not in the south's interests to have another war but if you want peace you have to prepare for war," he said.
Violent clashes between soldiers from the north and troops from the south in May in the disputed oil-rich border town of Abyei revealed how tense the situation had become.
The SPLM dreams of transforming itself from a group of guerrilla fighters into a proper, well-equipped modern army and is planning to establish its own air-force and navy to patrol the region's rivers.
The US and the UK both have programmes in the south to support the Sudan People's Liberation Army (SPLA).
This is not the first time the semi-autonomous region has received such equipment and experts say the north too is boosting its collection of heavy weapons.
In November 2007 military sources said that 35 Ukrainian tanks were transported to South Sudan after being offloaded at the Kenyan port of Mombassa.
The MV Faina was on its way to Mombassa when when it was hi-jacked off the coast of Somalia on 26 September.
There have been other reports of arms shipments over the last 12 months.
Under the 2005 peace deal, replenishment of ammunition, weapons and other military equipment is only allowed if it is approved by a north-south Joint Defence Board.
But senior SPLM official Yasir Arman said: "I believe that the SPLM has not violated any article of the Comprehensive Peace Agreement and I believe that the SPLM has nothing to do with dealing with arms.
"The SPLM is putting most of its money to development and services," he said.
Adding to the tension is the wait to see whether an arrest warrant will be issued against Mr Bashir on charges of genocide, war crimes and crimes against humanity for alleged atrocities committed in Darfur.
The Sudanese government is lobbying to have the United Nations Security Council postpone the request to issue a warrant.
An SPLM insider based in Khartoum said: "If it wasn't for the International Criminal Court then the north would have caused a lot of problems about the tanks but at the moment it's in their interest to stay silent."