Juba, Sudan (CNN) -- Nearing a January vote on independence, southern Sudan is acquiring its first military aircraft as the oil-rich region prepares for potential conflict in a nation Washington is calling a "ticking time-bomb."
Southern Sudan is a semi-autonomous region in Sudan that was granted self-rule under a 2005 peace deal ending a 21-year rebellion by the Sudan People's Liberation Movement (SPLM).
About 2 million died during the civil war, many from starvation. It was the second of two long conflicts between north and south Sudan since the nation's independence in 1956.
Under the 2005 accord, the southern region was promised a January 2011 referendum on whether to secede from the rest of Sudan and form a new country. Polls conducted by research groups indicate that southerners overwhelmingly favor separation.
On Wednesday, U.S. Secretary of State Hillary Clinton said that secession by Southern Sudan is "inevitable."
Speaking at the Council of Foreign Relations in Washington, she admitted "this is going to be a very hard decision for the north to accept" and described the situation in Sudan as a "ticking time-bomb."
Southern Sudan has agreed to terms with Russian supplier Kazan Helicopters to purchase 10 Mi-17 helicopters, according to a contract dating to May 2007 and finalized in March 2009, a copy of which was obtained by CNN.
The southern Sudan government was represented in the contract by the chief of staff of its military wing, the SPLA.
The first batch of four Mi-17-V5 "standard transport" choppers were to be flown to Entebbe airport in neighboring Uganda on August 12, according to correspondence between a senior SPLA official and a Kazan representative earlier that month.
This was a delay from an original May 2010 date, with the final batch supposed to arrive this month, according to the contract.
Southern Sudan officials deny the purchase.
The military spokesman for the north's Sudan Armed Forces (SAF) says it was aware that the south was acquiring 10 helicopters, but says that Khartoum was assured by the SPLM that the choppers were only for civilian use.
The 2005 peace deal required mutual consent of each party in order for either side re-equip itself, but both sides have been covertly adding to their military stores since 2005, analysts say.
But although both sides have participated in the contraband arms race, the north has far outpaced the south.
"While substantial expansion of state military capacity is occurring in both northern and southern Sudan, Small Arms Survey research has shown that the volume of the acquisitions and their level of sophistication are much greater in the north," said Claire McEvoy, the project manager for the Geneva-based group, which monitors arms flows.
The new aircraft mark an important step for the former rebels on their road toward becoming a professionalized armed force following the peace accord. Since its inception as a guerrilla insurgency in 1983, the SPLA has never had aerial capabilities.
"Generally speaking, the major disadvantage that the SPLA (south military wing) has faced has been its lack of air power. The SAF, (north forces) in contrast, has acquired a series of fighter aircraft and attack helicopters over the past years," McEvoy said.
According to data collected by IHS Jane's, a Colorado-based defense research group, the north military is believed to possess 43 attack fixed-wing planes -- as well as a dozen Antonov airplanes it used previously to drop bombs -- and 45 offensive helicopters.
The Mi-17 chopper being acquired by southern Sudan is primarily a transport helicopter, able to carry up to 36 troops at one time. It is deployed for both military and civilian use around the world, including by such countries as Afghanistan, China, and Venezuela. It is also a popular model for U.N. peacekeeping missions.
The Mi-17 can be modified to carry out air attacks, and can be outfitted with machine guns and rocket pods. It can also be used to drop 500 kg bombs, according to IHS Jane's.
"These appear to be the type of standard workhouse helicopters that are typical of the first phase in the building of an air fleet," says Lauren Gelfand, a Kenya-based analyst for IHS Jane's.
The south has made no secret about its desire for an air force. In 2008, the southern Sudan legislative assembly approved a "White Paper" on how to modernize its military that included plans for air forces.
In June, the south military spokesman Lt. Gen. Kuol Deim Kuol said in an interview in Juba that the army had trained pilots and ground engineers, and planned to have an air force before the 2011 referendum.
He denied this would violate the ceasefire arrangements of the peace deal, pointing to "nonlethal" support the U.S. and the U.K. governments are providing.
"The SPLA sees the armament process as being part of its ongoing professionalization, as part of which it is transforming itself from a guerrilla force into a conventional army that is capable of countering internal security threats and defending the south against outside aggression," McEvoy said.
Sudan's north-south border areas -- where most of Sudan's oil lies, mostly on the southern side -- remains militarized and tense, according to a report from Brussels-based International Crisis Group last week. The border has yet to be demarcated, and both governments rely heavily on the oil revenues derived from the area, the report said.
Preparations for the referendum have fallen drastically behind schedule, straining relations between the SPLM and President Omar al-Bashir's National Congress Party, the two parties that signed the 2005 accord. A complicated voter registration process has yet to begin.
"It's important to realize that south Sudan will need a legitimate standing military if secession goes forward in January as planned," Gelfand said.
"But the secretive nature of their procurement underscores the political turmoil that is accompanying the campaign for the referendum, and the south's perception of the very real threat posed by a Khartoum reluctant to accommodate the cleaving of the country," she said.
Of the ten helicopters, nine are Kazan's Mi-17-V5 model. The other is an Mi-172 "VIP" version that can carry up to 11 passengers in executive comfort.
The total cost of the the aircraft is $75 million, according to the contract.