In a Sudanese field, cluster bomb evidence proves just how deadly this war has become

The villagers of Angolo were gathered around the crater, mournfully staring at the bomb when we arrived. For over a month, the Russian-made cluster bomb has sat in the centre of this quiet farming village in Sudan's restive Nuba Mountains, its clutch of unexploded submunitions spilling from its belly into the red African soil. A makeshift attempt at cordoning off the scattered bomblets with a low circle of stones had little visible effect, with cattle and barefoot children moving unhindered through the long grass.

"Where is the West, where is the UN?" cried an elderly man, leaning against his spear as he gestured at the bomb. "How are we to clear this from our village? We need experts, and help from the outside world."

But South Kordofan's Nuba tribesmen have little hope of outside intervention. The UN's mandate to operate in Sudan's war-torn rebel provinces is heavily constrained by the government in Khartoum, and since South Sudan's declaration of independence in July 2011, little overt assistance is coming from their former allies a few dozen miles across the border.

Our minders from the SPLA-N, the Nuba guerrilla army fighting the Sudanese government, were keen to take us to Angolo, in the hope that that evidence of this bomb strike – the first ever confirmed use of cluster munitions in the South Kordofan conflict– might increase international pressure on Khartoum. Many quoted Libya as a model for intervention.

"When the West helped the thuwwar (revolutionaries) in Libya, they defeated Gaddafi in six months," says Abdul, a second lieutenant. "But we have been fighting the Arabs for 20 years with no help from anyone. Where is Obama, where is Merkel, where is Sarkozy?" News of the former French president's political demise has not yet reached this isolated mountain region.

Neither Sudan nor South Sudan is a signatory to the 2008 convention banning the use of cluster munitions, but the deployment of such indiscriminate weapons so close to civilian homes is widely considered a crime against international law. But, already wanted by the International Criminal Court for war crimes in Darfur, with a plummeting economy in Khartoum and a shrinking hold on the Nuba region, Sudan's embattled leader Omar al-Bashir may feel he has little left to lose.

"We are already in control of 75 per cent of the Nuba Mountains," said Abdul, as the sound of rebel artillery pounding the government-held town of Talodi echoed across the arid hills. "Now Bashir says he wants to come to pray in the mosque in Kauda. Kauda is our capital. Well tell him we are waiting, waiting for Bashir to come to pray," he laughed, "and we will hit him, and hit him all the way to Khartoum."

But the Angolo cluster bomb provides a mute rebuke to the SPLA-N's optimism. Without modern anti-aircraft weapons, the Nuba tribesmen have no answer to the Sudanese Air Force, which strikes the region daily with impunity. Local farmers have learnt to tell the difference between the slow lumbering throb of Antonov cargo aircraft, which drop unguided munitions from their rear ramps on the Nuba villages in contravention of international law, and the jet roar of MIG fighter bombers, which at least provide some degree of accuracy.

"Even just before you came here this morning, they bombed us," claimed a villager, who refused to give his name. "They bombed us in Abdey and the place called Jabrona, near here." Though The Independent heard an Antonov fly over the valley that morning, the villager's claims were impossible to verify, and indeed many aspects of the Angolo bomb strike remain mysterious.

The villagers said the air strike took place on 15 April, but were divided over whether an Antonov or MIG jet delivered the bomb. But the fact the bomb failed to explode at all is the greatest puzzle for munitions experts who have examined photographs of the incident.

The Angolo bomb is a Soviet-made RBK-500 cluster weapon, filled with dozens of spherical A0-2.5RTM submunitions, designed to burst in half on impact and scatter shards of shrapnel and ball-bearings over a wide area.

Each hemisphere of the bomblet is designed to achieve a "kill radius" of 20 metres, yet there were no reported casualties in the attack or after, and none of the submunitions appear to have exploded.

Perhaps the relative obsolescence of the bomb led to its malfunction. The serial numbers visible on the undeployed submunitions indicate that they were manufactured in Russia's Degtiarev plant in 1984.

Human Rights Watch observed a high failure rate for these submunitions in Russia's 2006 conflict with Georgia, though there is no known precedent for such a complete failure of this type of bomb in an airstrike. Even if the civilians of Angolo had a lucky escape, there were little grounds for optimism.

"There used to be many people in the Angolo, but many have gone to Yida [the UN-run camp just across the border struggling with a growing exodus of Nuba refugees] because of the bombing," said one.

Most of the village's straw-roofed huts seemed empty, with only a few women and children watching their menfolk at the bombsite from the safety of their mud-daubed compounds.

The terror effect of aerial bombing on South Kordofan's civilians has a brutal military logic. Like any insurgent army, the SPLA-N fighters rely on their civilian support base to grow and rear food, draw water from the abundant wells and gather firewood for fuel.

If the flood of civilians to refugee camps like Yida empties the region of its civilian population, the SPLA-N will be forced to draw supplies from South Sudan along the single vulnerable dirt track they still hold.

This narrow lifeline will become impassable when the already-overdue summer rains finally break, and convoys of military trucks were seen rushing equipment north, notably heavy-duty tyres.

The SPLA-N intend to hold their positions in the face of Bashir's much-advertised summer offensive, and hope to gain ground in the counter-offensive. Ordinary soldiers loudly protested their high morale.

"We shall go to the forests, and carry our guns, and we will liberate our country so that we can chase them back to Asia, where the Arabs come from," said Tariq, 23, a student turned fighter.

Then, shouldering his rifle, he fell back into line with his comrades as they trudged in single file across the dusty mountain path. As the soldiers marched, they passed empty hut after empty hut along the trail to their secret headquarters. Most of the villagers had long fled.

Channel 4 News will also report the story tonight at 7pm