South Sudan's army vowed Tuesday to hold their positions in a contested oil field seized from Khartoum's army, one week after the outbreak of bitter fighting that has raised fears of a wider war.

Despite air strikes and a reported counter-attack by Khartoum's Sudan Armed Forces (SAF) to retake the disputed Heglig oil field, the South's Sudan Peoples Liberation Army (SPLA) said it would not withdraw from the battle zone.

"If they advance, definitely SPLA is ready to fight back and repulse them... the SPLA is ready for them outside Heglig," Southern army spokesman Philip Aguer told reporters.

The hostilities are the worst since South Sudan's independence from Sudan in July, and world powers have condemned the fighting, as fears grow clashes could spread beyond the current border conflict.

Fighting broke out last month between Khartoum and Juba in the Heglig oil field -- key to Sudan's already struggling economy, as it supplied around half of its oil production -- before an escalation of violence last Tuesday.

The South has reportedly placed tanks and artillery around oil infrastructure in Heglig, but as Sudan does not allow journalists to report independently in South Kordofan state to which Heglig belongs, it is difficult to verify the military situation.

Aguer said Khartoum had damaged wells as they sought to dislodge Southern troops by aerial bombardment.

"The border is still fragile, tension is still very high," Aguer added. "The SAF continue to bomb indiscriminately... On Tuesday they bombed one of the oil wells outside Heglig, it is still burning."

Khartoum has launched a wave of air raids on Southern border areas, killing several civilians and hitting a United Nations peacekeeper base on Monday in the village of Mayom, in the South's oil producing Unity state.

The attack was confirmed by the UN, although Sudan denied the air strike.

However, the region appeared calmer Tuesday, with "no reports of fighting so far," said Gideon Gatpan, Unity state's information minister.