* UN, aid groups barred from Sudan conflict zones
* Sudan accuses South Sudan of backing rebels
* US says Khartoum continues to bombard civilians
By Louis Charbonneau
UNITED NATIONS, Jan 8 (Reuters) - The United Nations warned on Tuesday of a worsening humanitarian crisis in Sudan's conflict-torn southern region, amid reports that people there were starving to death and others were surviving on roots and leaves.
"This is 2013 and to think that tens of thousands, hundreds of thousands of people are living in such desperate and deplorable circumstances, and we can't get in to help them as humanitarian organizations, it's just not acceptable," John Ging, a senior U.N. humanitarian official, told reporters.
He was referring to the situation in the southern Sudanese states of South Kordofan and Blue Nile, which have been racked with conflict for more than a year, causing hundreds of thousands of people to flee their homes.
Rebels in the area, known as SPLM-North, say they are fighting to protect ethnic minorities from repression and marginalization, while Khartoum accuses them of trying to spread chaos at the bidding of South Sudan, which seceded last year.
Fighting began in South Kordofan in June 2011, just before South Sudan seceded from the north, and spread to Blue Nile in September of the same year.
U.N. and other humanitarian aid agencies say they have been barred from the region by Khartoum, which denies the charge and insists there is no humanitarian crisis there.
Ging said that those who escaped from the area spoke of people "relying on roots and leaves."
"We're not allowed in to do the assessments," Ging said after briefing the 15-nation U.N. Security Council behind closed doors. "I do have to communicate what we're being told by the people who are fleeing ... People are dying in South Kordofan, that is what we are told."
"When we look at the emaciated state of the children and adults who have successfully made the journey out of these two areas, we can see in their physical state the obvious suffering they have endured for a very long time that has resulted in them being in this appalling condition," he added.
South Sudan seceded from Sudan under a 2005 peace deal that ended decades of civil war, but both countries have yet to agree on ownership of several disputed border regions.
U.S. Ambassador to the United Nations Susan Rice said Washington remained "deeply concerned by the continued deterioration of the humanitarian situation" in the two states.
"We're also deeply concerned about the ongoing aerial bombardments by the Sudanese Armed Forces, including in civilian areas," she said.
Rice called on Khartoum and the SPLM-North to grant aid agencies access to South Kordofan and Blue Nile.
Speaking to reporters after the council meeting, Sudan's U.N. Ambassador Daffa-Alla Elhag Ali Osman reiterated his allegations that South Sudan was supporting the rebels in South Kordofan and Blue Nile states.
Sudan and South Sudan are under threat of Security Council sanctions if they fail to resolve their remaining disputes and settle on a comprehensive peace plan.
South Sudan said on Monday it hoped to establish a demilitarized zone along its border with arch-rival Sudan within a month, paving the way for vital oil exports to resume.
Sudanese President Omar Hassan al-Bashir and South Sudan's Salva Kiir met in Ethiopia over the weekend to try to quell tensions that have rumbled since a flare-up of violence along the disputed border last year, the worst since the South seceded.