On South Sudan's tense front line with Sudan, Southern troops lie in the shade of trees alongside the tanks hidden there from the spying eyes of Sudanese warplanes overhead.

"They bombed here 24 times on Monday night," says Brigadier General James Kuac, searching the sky in the fading evening light for the bombers that have unleashed a series of deadly air strikes on the South's Unity state.

After bloody battles between the rival armies this month, the frontline has been calmer in recent days, since Southern troops repulsed a heavy attack at this forward outpost by Khartoum's Sudan Armed Forces (SAF) on Sunday.

But the troops remain on full alert, never straying too far from the long lines of scattered foxholes dug into the hard baked earth.

In a haze of orange dust, tanks manoeuvre and khaki-clad figures set out on reconnaissance missions the few kilometers (miles) forward to Teshwin, where they say Sudan troops have likewise dug into fortified positions.

Clashes erupted on April 10 when Southern troops seized the contested Heglig oil fields from Sudanese soldiers, an area that both nations claim lies on their side of the undemarcated border.

South Sudan pulled back to Teshwin ten days later, claiming it bowed to international pressure and fears that their newly independent country could slip back to all out war with former civil war foe Sudan.

But South Sudan says despite the pullout, the attacks continue.

Earlier this week Sudanese fighter jets hit the key town of Bentiu -- capital of the South's oil rich Unity state -- despite the South's pull-out, dashing hopes an escalating conflict could be cooled.

At a market just outside Bentiu town, charred and twisted metal mark the remains of shops and stalls destroyed by the airstrike, in which a teenager was killed, provoking anger in the people in the town.

"This is the consequence (of withdrawal from Heglig), we have brought the war home," said Unity Governor Taban Deng.

As flames consumed the market stalls, people dragged out the charred body of the boy, curled up tight in death as though to protect himself.

"The boy was trying to run and save himself, and he lay down to try and escape the bomb," says Teresa Nyakuoth, a neighbour who saw the fierce blast.

With fumbling hands, men tried to hang a sheet over the child's bent knees and arms to cover the horror.

His back to the ashes, the boy's father Duop Wich said the family had brought their cattle to auction, and that he had sent his 15-year-old son Bil to buy biscuits and sugar on his way home.

"I cannot describe anything about my son because I have lost him," said Wich, looking into the distance as his eyes tightened.

"He was in school, and he wanted to be a scientist," he mumbled softly, staring at the patchwork of litter trodden in the dusty earth.

Hours after the attack, the family of another teenage boy wounded in the air strike tried to stop him convulsing on a bloodied bed by holding down those parts of his arms not peeled pink from the bomb. He died later that day.

Soldiers are bolstering defences should -- as many fear -- Sudanese troops try to push across the border to attack the South's oil fields, and seize the town of Bentiu.

The road from Bentiu to the frontline now appears relatively quiet, but soldiers say their defensive lines stretch deep into the bush.

"The oil was a blessing, but now it is a curse", says William Makuach, a teacher in the town. "Wherever there is oil, Sudan will come and say it is theirs."

By a reed fence flattened by the impact, Nyakuoth says that many of her neighbours have fled, and local shopkeepers speak of their fear of working in what was previously a bustling market.

Wich worries that Sudanese forces could come to Bentiu, but says that after such a struggle for the South's independence -- which broke free in July after a two decade long war -- he will leave only if forced.

"Whether it finishes by negotiation or war, that day will come -- but it will be a long process."