* Rebels accuse Sudan of bombing civilians, Khartoum denies
* South Kordofan is another hotspot in flickering border war
* Hundreds hide from fighting in rocky caves in mountains
By Hereward Holland
NUBA MOUNTAINS, Sudan, May 10 (Reuters) - Leaning on a hillside rock with her child balanced on her one sound leg, Nafisa Juad looks out at the roofless ghost town of Buram on the dusty plain below.
The scorched and ruined homes of Buram, a now deserted settlement amid the Nuba mountains of Sudan's South Kordofan state, bear testimony to vicious clashes in January between Sudanese government armed forces and the rebel Sudan People's Liberation Movement North (SPLM-N).
Since the latest round of fighting erupted in June last year, the combat and what local leaders describe as sustained bombings by Sudanese warplanes have driven tens of thousands of civilians like Juad into caves and rocky shelters in the hills.
"I was running from the sound of the Antonov (aeroplane), carrying my baby, when the bombs dropped and cut my leg," she said, recounting the moment four months ago when most of her left leg was blown off in an attack. The scarred stump protrudes from beneath her burgundy-coloured dress.
The fighting in South Kordofan is yet another pocket of conflict flaring on the disputed border with southern neighbour South Sudan, which became an independent state last year.
Last month, the two countries' armies skirmished over a border oil area, raising fears they would plunge back into a full-scale confrontation, like the civil war Sudan's north and south fought for more than two decades before a 2005 peace deal.
Rebellions in South Kordofan and Blue Nile states, which remained within Sudan's territory after the July 2011 secession of South Sudan, and also insurgents in the western Sudanese region of Darfur, are still stoking the chronic instability that has plagued Sudan since its independence from Britain in 1956.
This has pitted groups on the periphery of the nation against a central Arab governing political elite in Khartoum whose critics say has marginalized many of Sudan's peoples.
Civil society leaders in the Nuba Mountains, a jigsaw of communities mixing Muslims, Christians and others practising traditional African beliefs, estimate that out of the 350,000 displaced by the fighting as many as 100,000 are hiding in caves, eating tree leaves, sap and wild fruits. They say some are starting to die from hunger.
"There is no food, this is what we eat," Juad said, displaying a tin bowl of chopped leaves and dry seeds.
"Since the war started, the people have been terrified, living in caves. There's no way to grow anything or graze our cows ... nothing is here," said Ahmed Tia, a local commissioner of Buram county, sitting on a leather office chair under a tree.
As an Antonov aircraft buzzed overhead, Tia accused Sudan's government of carrying out the bombings to drive people from their farms. He said Khartoum used hunger as a weapon of war.
The SPLM-N says the Sudanese army employs this tactic to prevent people from growing crops that could be used to support the insurgencies in South Kordofan and Blue Nile.
Sudanese military spokesmen deny such accusations, and frequently deny that any bombing raids take place.
"The rebels should accuse themselves of these violations. This is not the behaviour of our governnment," Rabie Abdelatie, an official from Sudan's ministry of information, told Reuters.
The United Nations is unable to fully verify the humanitarian situation in South Kordofan because of limited access granted by Sudan to it and foreign aid agencies.
REBEL ALLIANCE, EXTENDED FRONT
Despite Khartoum's denials, New York-based Human Rights Watch has accused the Sudan's military of conducting indiscriminate bombings against civilians in the Nuba Mountains.
"Such attacks may amount to war crimes and crimes against humanity," it said in a statement.
During Sudan's more than two-decade civil war that ended in 2005, the SPLM-N fought as the 9th and 10th divisions of the rebel alliance that now rules in South Sudan.
Six years after the peace deal, the war reignited in South Kordofan in June 2011 and later in Blue Nile when Sudanese troops tried to forcefully disarm the rebels.
Khartoum accuses South Sudan's government of giving the SPLM-N military and financial support, as well as sanctuary. Juba firmly denies the charges and SPLM-N leaders say they captured their weapons and vehicles from government forces.
Increasing the rebel pressure on Sudanese President Omar Hassan al-Bashir in Khartoum, the SPLM-N late last year joined with three insurgent groups from Darfur and senior figures in two main Sudanese opposition political parties to form the Sudan Revolutionary Front (SRF) alliance.
The SRF has faced ridicule from Sudan's government, which said it had no chance against the Sudanese army. Sceptical analysts have seen Sudanese insurgents rise and fall before.
But this does not dent the confidence of the SRF's chief of staff, Abdelaziz Alhilu.
In an interview with Reuters at his Nuba Mountains headquarters, Alhilu said the anti-Bashir alliance meant the Sudanese president's troops faced enemies from the Ethiopian border in the east to the frontier with Chad in the west, a front of around 2,000 km (1,250 miles).
"This is a very long frontline. We are now attracting SAF (Sudanese army) to our areas, bogging them down, pinning them down," he said. He added the SRF was using guerrilla tactics bolstered by captured tanks and pickups mounted with heavy weapons. Some were shown to visiting Reuters reporters.
"We are optimistic but also realistic. We cannot say tomorrow but we are sure we are going to reach Khartoum in the shortest possible time, maybe in one year's time," Alhilu said.
But the rebels face a 700km (440 mile) slog to Khartoum across terrain generally flatter than their mountain strongholds, which could favour Sudan's better-equipped army.
The SRF says it wants to hold a conference involving Sudan's political parties to draft a new constitution that would then allow for a peaceful exchange of power.
Sudanese spokesman Abdelatie scoffed at Alhilu and the SRF's ambitions. "Let him try. This claim is a dream. He is dreaming while he's awake," he told Reuters.
RAINS ARE "GUERRILLA TIME"
Although such claims are impossible to verify in the vast, remote area, Alhilu said his SPLM-N now controls 90 percent of the countryside of the Nuba Mountains. He says only the towns of Kadugli and Talodi remain under SAF control.
Bashir, who is wanted for war crimes and genocide by the International Criminal Court, visited Talodi last week and said his forces would take over the nearby rebel stronghold of Kauda within a week. Analysts said this was unlikely.
"(Both statements) are bluster but it's also at the core of what they believe and why this is such an enduring conflict," Sharath Srinivasan, director of the centre of governance and human rights at Cambridge University told Reuters.
Driven from their homes and land by the fighting, and facing starvation in their cave hideouts, hundreds of Nubans are making the dusty trek over the border to the swelling Yida refugee camp 10 km (six miles) inside South Sudan.
More than 700 arrived on one day last week, some walking through the night with their possessions perched on their heads.
Aid agencies running the camp say a bad situation is about to get worse with the onset of the rainy season.
"People get dysentery and malaria without access to medical care ... We doubled our admission of severely malnourished children in April," said Kelly Nau, a nutritional coordinator for Samaritans Purse, a Christian aid agency working in Yida.
But for the SPLM-N rebels, the rainy season heralds a chance for an offensive. Historically, the Sudanese army's superior firepower has given it an advantage in the dry season but its heavy weaponry and vehicles become bogged down when rain turns the countryside's fine dust to thick mud.
"The rainy season is guerrilla time," said Jagod Mukwar, an ebullient SPLM-N general in charge of the eastern division, speaking in a straw hut near the front line not far from Talodi. (Editing by Pascal Fletcher and Andrew Heavens)