Pointing to a scarred bald patch above his ear, Lise Dide shows where shrapnel grazed his head when his village in Sudan's Blue Nile state was hit in an air strike.

The plane came when I was asleep. I was still in my bed, I did not hear the sound, he said in South Sudan's Doro refugee camp, set up just three weeks ago some 40 km (25 miles) from the Sudanese border.

I woke up to the sound of the bombs. I tried to get up and put on my T-shirt. Immediately the bomb exploded, I got hit in the head and I fell down, he said.

Dide is one of more than 80,000 Sudanese that have sought refuge in South Sudan from clashes between government forces and insurgents on the northern side of the poorly-marked and tense border, according to the United Nations.

In the border states of South Kordofan and Blue Nile, Sudan's army is fighting insurgents who were allied with southern rebels during decades of civil war that culminated in South Sudan's secession in July under a 2005 peace agreement.

More than 410,000 people have been displaced in the two states, according to the latest U.N. figures.

Faltering negotiations between Juba and Khartoum over post-secession issues such as oil, debt, disputed areas and transitional finance as well as a military build-up on both sides of the border risks creating further instability.

Some 650 people follow in Dide's footsteps across the border every day, U.N. officials say, many of them veterans of displacement, having lived in Ethiopian camps throughout the civil war and returning home after the 2005 peace deal.

One refugee said God must have created the word refugee for the Sudanese.

SEARCHING FOR FOOD

Across the border from Doro Camp, the land is controlled by the Sudan People's Liberation Movement North (SPLM-N), which aims to topple President Omar Hassan al-Bashir in Khartoum. Many in the refugee camp are related to SPLM-N fighters.

Khartoum accuses Juba of supporting the rebels, which the ruling party in South Sudan denies. In turn, SPLM-N spokesman Arnu Ngutula Lodi has said the Sudanese army is bombing civilian areas to deprive his guerrilla army of food and support. Those charges are denied by Khartoum.

It is difficult to verify Lodi's claims because Khartoum does not give access to conflict areas in Blue Nile and South Kordofan to U.N. agencies, aid groups or journalists.

However, numerous refugees interviewed by Reuters during a five-day visit to the camp said their villages were attacked by Sudanese ground forces or Antonov planes.

Refugee Lamin Muhammed Yazin fled his home in Kurmuk when the army attacked but returned later to search for food.

They destroyed everything; they killed the goats, they killed the chickens and inside the house where we have sorghum and maize, they brought it outside, destroyed it and scattered it, he said.

Cradling her one-day-old baby in a mosquito net hung from twigs, Nyakoma Ruben said she walked heavily pregnant for a month to reach Doro.

It was so difficult. I could walk for only one hour and then I had to rest because I quickly became tired, Ruben said.

Unable to carry enough food for the journey, many refugees said they ate bitter wild berries and the roots of trees. One woman sold her dresses to buy her children provisions.

Conditions in the camp aren't much easier. Ruben's husband missed the birth of her child Abraham because a lack of food forced him to return to his farm in Blue Nile.

Women queue through the night to draw water from hand-pumps and the local clinic is overwhelmed.

When people arrive, they are arriving tired, and they have already lost lots of energy by walking. Their body's immunity is not at its best, said Medicins Sans Frontieres emergency coordinator Asaad Kadhum.

Malaria, respiratory tract infections and diarrhoea are the leading diagnosis in MSF's emergency clinic that is treating over 100 people per day, he said.

As numbers at the camp swell, aid agencies expect the situation to deteriorate.

The U.N. refugee agency says a group of 10,000 refugees were recently identified near the border town of Elfoj, and thousands more are believed to be stranded in remote locations along the border.

(Reporting by Hereward Holland; Editing by Ulf Laessing and Sonya Hepinstall)