* Says constructing South Sudan is work of lifetime
* Sees relationship with Sudan as biggest challenge
* South Sudan can "forgive but not forget" war years
By Pascal Fletcher
April 23 (Reuters) - South Sudan's President Salva Kiir, who spent much of his life as a rebel commander fighting in one of Africa's longest and deadliest civil wars, says it will take another lifetime to make his newborn country prosperous, secure and self-sufficient.
For now it is locked in fighting with its arch foe Sudan, the worst violence since South Sudan became independent under a 2005 peace agreement with Khartoum.
"Building a nation will take our lifetimes," the guerrilla fighter turned president told leaders of his ruling Sudan Peoples' Liberation Movement (SPLM) last month, listing huge challenges in infrastructure, education and healthcare.
While other members of the southern elite boast academic credentials obtained in the West, Kiir is seen as a no-nonsense army man, most comfortable in the field. He joined the south's first insurgency (1955-1972) at 17 and later became a major in the Sudanese intelligence services.
In a tough speech to parliament interrupted by clapping, Kiir asked his people in April to prepare for war after his troops made a surprise grab of the disputed Heglig oilfield.
He admonished U.N. Secretary-General Ban Ki-Moon for asking him to leave Heglig, which is key to Sudan's economy. "I told him you don't need to order me because I am not under your command," Kiir said.
Bowing to demands from the U.N. Security Council, South Sudan said on Sunday it had withdrawn its troops from the contested Heglig oil region, raising hopes the neighbours had pulled back from the brink of all-out war.
While promising to work for national unity and reconciliation, Kiir warned South Sudanese in his March 26 speech that "the biggest challenge to achieving this vision is our relationship with the government of Sudan".
Following South Sudan's birth as Africa's newest state in July 2011 after a landslide southern referendum endorsing this move, Kiir's SPLM government has quickly become embroiled this year in disputes with Khartoum over ill-defined border lines, and especially over the economic lifeblood of both states: oil.
Independence gave landlocked South Sudan three-quarters of the oil output previously held by Sudan. But Juba has rejected Khartoum's insistence, reinforced by confiscation of southern cargoes, that it pay a certain level of transit fees to send its oil through Sudan's northern pipelines and Red Sea port.
Accusing Khartoum of "stealing our oil", South Sudan in January shut down its entire output of 350,000 barrels a day, a potentially crippling move for both economies.
Kiir, a committed Catholic with a reputation as a conciliator, said he would not give in to what he called Khartoum's strong-arm tactics. He said the South would seek to develop without crude revenues - a Herculean task for a nation where oil provides 98 percent of state revenue.
"It is difficult for me to accept a deal that leaves our people vulnerable, dependent and paying billions they do not owe," he said about the unresolved oil dispute with Khartoum.
"FORGIVE, BUT NOT FORGET"
Kiir, one of the founders of the SPLM who took over as its leader after the 2005 death in a helicopter crash of the charismatic John Garang, has repeatedly said he does not want to go back to war with Sudan. Before this month's clashes, Sudan's President Omar Hassan al-Bashir made similar statements.
It is clear that years of conflict between South Sudanese, who are mostly Christian or follow traditional religions, and the government, military and militias of the largely Muslim and Arabic-speaking north have left deep scars.
"We have been bombed, maimed, enslaved and treated worse than refugees in our own country, but we have to forgive, although we will not forget," Kiir said at the moment of independence last year, the culmination of a 2005 peace deal that ended more than two decades of north-south war.
He had invited Bashir to a meeting in Juba on April 3 before the fighting at Heglig broke out when South Sudan occupied the zone in response to what it said was Khartoum's attacks.
When Kiir first stepped up as the leader of the then semi- autonomous south in 2005, many compared him unfavourably with his firebrand predecessor, the civil war hero Garang.
But Kiir, a former major who rarely gives interviews, has concentrated on keeping his fragmented territory united. He has repeatedly offered amnesties to renegade militia leaders in the south and made progress in fighting rebels.
Kiir has been good at bringing old foes into the SPLM and the southern army. But worries remain about the ability of his party to accept rivals who insist on staying outside the fold.
His quiet approach and lack of polarising rhetoric are seen as his main strengths in governing a landlocked territory handicapped by tribal divisions, severe poverty, unstable neighbours and huge supplies of privately held weapons.
Thousands of people have died inside South Sudan in recent years in tribal clashes. The south says northern-backed militias provoked the violence. Khartoum denies the accusation.
Kiir is a member of the country's largest tribe, the Dinka, which largely controls the army and government, to the dismay of smaller tribes.
North and South Sudan's armies are also still facing off in flashpoints along their ill-defined shared border - including in Southern Kordofan, South Blue Nile and the contested Abyei area.
On Monday residents and officials in the capital of South Sudan's Unity State said Sudanese war planes bombed a market there. The North denied carrying out the raid, which was witnessed by a Reuters journalist.
Kiir says he wants to turn South Sudan into "a land of promise - not a land of conflict, fragility and disaster".
But as he told an audience recently: "We are starting from zero and starting with very little". (Writing by Pascal Fletcher; editing by Philippa Fletcher)
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