Sudan on Tuesday carried out new airstrikes inside South Sudan, as rival armies exchanged artillery fire in the latest round of fierce fighting in contested border regions.

An AFP correspondent in the South Sudanese frontline village of Tashwin heard heavy artillery shelling and multiple airstrikes lasting for around an hour, with one bomb dropped by aircraft landing less than a kilometre (mile) away.

Khartoum vowed late Tuesday to react with "all means" against a three-pronged attack it said South Sudanese forces had launched against South Kordofan state, including its key oil-producing region of Heglig.

A statement on the official SUNA news agency warned of "destruction" in South Sudan.

The bombing follows border fighting that erupted two weeks ago between the two neighbours, the most serious unrest since Juba's independence last July, and prompted international fears of a return to full-blown conflict.

Southern Information Minister Barnaba Marial Benjamin said that Sudanese airplanes "bombarded Abiemnom, well within the sovereign territory of South Sudan."

"Initial reports confirmed that four civilians have been wounded, including a small child," Benjamin told reporters in the Southern capital Juba.

Clashes last month broke out along the undemarcated and disputed frontier in the Heglig area, with each side blaming the other for starting the fighting.

The dusty village of Abiemnom in Unity state is some 40 kilometres (25 miles) south of the border with Sudan, but also lies on a strategic road to the contested Abyei region, some 10 kilometres (six miles) away to the west.

"The intended target was a strategic bridge in Abiemnom," leading to Abyei, Benjamin added.

The statement on SUNA claimed that South Sudanese forces were backed by rebel groups. It did not specify which rebels but the Sudan People's Liberation Movement-North (SPLM-N) has been battling government troops in South Kordofan since June.

Sudan has also previously alleged that Darfuri rebels were operating in the Heglig area.

South Sudan has denied backing opposition groups in the north.

African Union mediator Thabo Mbeki held talks late last week over the crisis with South Sudan's President Salva Kiir and his Sudanese counterpart Omar al-Bashir, but tensions remain high between the two sides.

Benjamin claimed northern troops, backed by tanks and proxy militia forces, were advancing towards the South's Unity state, where key oilfields are located.

"Two brigades of Sudan Armed Forces, backed by 16 tanks and accompanied by members of the mujahedeen and other militia loyal to Khartoum, are currently moving towards Unity state with the intent to capture and occupy the oilfields," he added.

"The Republic of South Sudan condemns the bombardment of innocent civilians, and calls on Sudan to immediately withdraw from the sovereign territory of South Sudan."

He did not clarify whether northern troops had crossed the border.

Large Southern Sudanese troops movements were seen close to the frontier, with convoys heading up to the frontline.

Despite the violence on the border, Juba ordered Tuesday that Sudanese nationals living in the newly independent country be treated with respect, after a deadline requiring them to formalise their status expired.

"All nationals of the Republic of Sudan are declared foreigners as of 9 April 2012," South Sudanese Interior Minister Alison Magaya said in a statement.

"Sudanese nationals shall be accorded fair treatment and full respect in regard to their human rights."

An April 8 time limit ended a grace period after South Sudan separated in the wake of an overwhelming "yes" vote in an independence referendum that followed Africa's longest civil war.

Tens of thousands of Sudanese are believed to live in the South -- although the exact figure is not known -- significantly fewer than Southerners in Sudan.

More than 370,000 Southerners have returned from Sudan since October 2010, but an estimated 500,000 others remain in the north.

Those seeking to apply for northern residence need documents from South Sudan but many cannot afford a trip South to get the relevant papers.

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