Border battles threaten the new Sudans
The steady, low drone of the Antonov over their temporary shelters sent the refugees running - and announced a dangerous new phase in Sudan-South Sudan relations.
Despite Khartoum's denials, Juba is convinced Thursday's bombing of the refugee camp in Yida, in South Sudan's Unity State, was the work of the Sudanese military.
It is reported that several people were killed in another aerial bombardment the next day in Upper Nile, also in South Sudan.
The war of words has come a step closer to a genuine conflict, in the tense lands either side of the still not completely defined new international frontier.
In the last few days South Sudan's President Salva Kiir has accused Sudan of wanting to drag his country back into a "meaningless war".
The new government in Juba believes that Sudan's President Omar al-Bashir is supporting southern rebels behind periodic attacks near oilfields in Unity State and Upper Nile.
"Omar al-Bashir believes only in war. He has problems in the north so he is trying to shift this to South Sudan to cover his weakness in his own country," Maj Gen Mangar Buong of the South Sudan army told the BBC.
Sudan denies this and has repeated its accusation that Juba is supporting rebels north of the border in the states of South Kordofan and Blue Nile.
It is very hard to prove any of the competing claims, but the impact of the fighting on people is clear to see.
Stephen Gatwech is only five or six years old, with infectious laughter and a big smile. His loves his new toy, a trumpet made from shiny paper.
He is also missing most of his left leg - the result of a landmine accident. The mine, laid by rebels in Unity State fighting the southern government, reduced his foot to slivers of useless skin and killed his grandmother and others.
The mines and the occasional rebel attack are making life in Unity State difficult - as is Sudan's decision not to let many goods cross the border into South Sudan.
"I cannot say the economic situation in Unity State is good," says James, who owns a pharmacy in Bentiu, the state capital. "It's because the road from Sudan has been closed. The prices are really going up."
Others complain about their fear of travelling because of the landmines.
The South Sudan Liberation Army (SSLA) rebels in Unity say they are fighting against corruption, underdevelopment and mismanagement of the oil revenues, and liken South Sudan's leaders to the mafia and former Ugandan dictator Idi Amin.
And it is surely no coincidence the rebels are operating where South Sudan's oil reserves are - the southern army spokesman has even called this a "war about oil".
In Upper Nile, southern rebels have attacked the army and now the charity Oxfam has pulled its staff out of the state because it said they were too near aerial bombardments over the border and a reported troop build-up.
'Rest and recuperate'
There are probably similar stories to be told over the border in South Kordofan and Blue Nile, but Sudan is not keen to let journalists visit these sensitive areas.
The situation up and down the border seems to be getting worse, just four months after South Sudan became independent.
Meanwhile, the status of the disputed territory of Abyei remains unresolved, though it has been in the hands of the Sudanese Armed Forces since May.
But if Sudan did bomb several places in South Sudan it is now operating militarily across an international border, an extremely serious act.
The same judgment would apply if South Sudan was found to be supporting SPLM-North rebels in Blue Nile and South Kordofan.
People fleeing fighting in those states have crossed the border to the south, and it appears the areas where they have gathered were bombed.
Khartoum believes it is the northern rebels who cross into South Sudan to rest and recuperate, before rejoining the fray.
That seems to be the logic behind the Sudan foreign ministry spokesman's declaration that there are no refugee camps in South Sudan.
The camp at Yida might contain SPLM-North fighters, though they deny this, but I also saw many women and children and old men, who clearly were not combat personnel.
Whatever the truth of the matter, there is no doubt the incident has further undermined Sudan and South Sudan's already fragile relationship.
The UN and the US have both already condemned Sudan, and the flurry of denials from Sudanese officials suggest they must have at least some concerns about the damage the accusations are doing to their country's standing.
Discussions in the UN Security Council also highlight the ongoing conflict either side of the border.
President Kiir has made it clear he will not let South Sudan return to war. But the increased tension cannot help the ongoing negotiations on post-secession matters, and the situation could potentially degenerate much further still.
The failure to resolve a number of issues before South Sudan seceded, including the border, security and oil, is being paid for now; and above all the lack of a meaningful solution for the areas of Abyei, South Kordofan and Blue Nile.
These were, sadly, all too predictable sources of conflict. Now the virus seems to be spreading.
Both Sudan and the South are reliant on their oil revenues, which account for 98% of South Sudan's budget. But the two countries cannot agree how to divide the oil wealth of the former united state. Some 75% of the oil lies in the South but all the pipelines run north. It is feared that disputes over oil could lead the two neighbours to return to war.
Although they were united for many years, the two Sudans were always very different. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
The health inequalities in Sudan are illustrated by infant mortality rates. In South Sudan, one in 10 children die before their first birthday. Whereas in the more developed northern states, such as Gezira and White Nile, half of those children would be expected to survive.
The gulf in water resources between north and south is stark. In Khartoum, River Nile, and Gezira states, two-thirds of people have access to piped drinking water and pit latrines. In the south, boreholes and unprotected wells are the main drinking sources. More than 80% of southerners have no toilet facilities whatsoever.
Throughout the two Sudans, access to primary school education is strongly linked to household earnings. In the poorest parts of the south, less than 1% of children finish primary school. Whereas in the wealthier north, up to 50% of children complete primary level education.
Conflict and poverty are the main causes of food insecurity in both countries. In Sudan, many of the residents of war-affected Darfur and the border states of Blue Nile and South Kordofan, depend on food aid. The UN said about 2.8m people in South Sudan would require food aid in 2013. The northern states tend to be wealthier, more urbanised and less reliant on agriculture.