* Carter says most, maybe all, Sudan debt should be forgiven
* Violence blocks returning southerners: aid sources
* Referendum turnout passes key 60 pct threshold: commission
(Updates with confirmation of turnout threshold)
By Jeremy Clarke
JUBA, Sudan, Jan 13 (Reuters) - South Sudan's independence referendum will meet international standards, former U.S. President Jimmy Carter, who is observing the plebiscite, said in the first significant endorsement of the vote.
Southerners are widely expected to choose to declare independence from the north in the week-long poll that entered its fifth day on Thursday -- a plebiscite promised in a 2005 peace deal that ended decades of north-south civil war.
Carter told journalists on Thursday he also believed that international creditors should forgive most if not all of Sudan's crippling debt, to give both north and south access to fresh funds after the referendum.
The referendum's organising commission on Thursday said the vote's turnout had passed the 60 percent minimum needed to make the result binding, confirming its earlier forecasts.
Aid groups said thousands of south Sudanese trying to get home for the poll had been left stranded and dependent on food aid in the north after logistical problems and attacks in border areas blocked their route.
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For a story on the peace process click on [nMCD238256]
For an analysis on the south click on [nHEA155011]
For more on the referendum click on [nLDE70708L]
For interactive timeline http://link.reuters.com/cew55r
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"I believe it will meet international standards on the conduct of the process and also the freedom with which people have cast their votes," said the former president, whose Carter Center is running one of the vote's largest teams of observers.
"I don't think there is any doubt the results will be accepted without serious challenge."
Many northerners are angry about the referendum, which looks likely to deprive them of a quarter of the territory in Africa's largest country and most of its known oil reserves.
DEBT BURDEN
"My belief (is) that a substantial portion -- perhaps all -- (of) existing debt should be forgiven because both the north and south need to have a net inflow of funds in the future to make it possible for them to sustain the shock that will come from separating the two nations," Carter said.
"There is about 38 maybe 39 billion dollars debt now and the World Bank tells us about 30 billion of that is in arrears. That is a very serious burden of debt. I have a list of the creditors to whom the debt is owed and can't see anybody on the list who can't afford to forgive it."
Highlighting tensions, aid sources said Sudanese security forces had closed road and rail links passing north and west of the scene of recent clashes between Arab nomads and southern police and civilians in contested border regions.
At least 46 people have been killed since Friday, according to reports from both sides.
"The challenge is not knowing if or when hostilities will sufficiently cease to allow routes to reopen or whether it will continue, at least sporadically," said one aid official.
Fighting has followed increasing friction over the unresolved status of the fertile and oil-producing central region of Abyei, claimed by both Arab Misseriya nomads and the Dinka Ngok people, who are associated with the south.
More than 600 southerners were stuck in Kadugli, the capital of Southern Kordofan state, which surrounds Abyei, after armed men attacked parts of their convoy on Monday, said an official from the International Organisation for Migration (IOM).
Aid officials said more than 3,000 people -- two thirds of them children -- were stranded in Kosti, an important way station in the north's White Nile state.
A total of 25,000 southerners had already returned to Abyei, according to the United Nations World Food Programme. "We are expecting another 4,000 in the next few days but because of the insecurity on the route we are not sure if they will come or not," said senior WFP official Mahadevan Ramachandran.
Students clashed with police in two cities in north Sudan on Wednesday in a rare public protest over price rises, sparked by an economic crisis that has been exacerbated by fears of the economic impact of the south's secession. [nMCD335630]
The vote's turnout passed its key 60 percent threshold after just four days of voting, said the deputy chairman of the referendum commission Chan Reek Madut.
"At the end of the fourth day of polling, with 86 percent of referendum centres reporting, 2,360,922 people are confirmed to have voted in southern Sudan. This exceeds the 60 percent threshold," Madut told reporters.
Preliminary results of the referendum are to be announced at the start of February, with official results two weeks later.
(Additional reporting by Andrew Heavens in Khartoum; editing by Giles Elgood)
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