to transit the oil.

Each side blames the other for starting the recent fighting along the undemarcated and disputed frontier in the oil-producing Heglig area.

A brief statement from Sudan's ministry of information said its Heglig oil fields were deliberately targeted in the attack.

Villagers 'injured'

South Sudan's military spokesman, Philip Aguer, told the BBC the army had repulsed Sudan's soldiers all the way to Heglig in response to earlier attacks from the air and the ground inside South Sudan.

But he did not confirm whether the southern army had attacked oil installations inside Sudan.

According to South Sudan's information minister, four civilians, including a child, were wounded earlier when the village of Abiemnom was attacked by Sudanese planes in Unity state, about 40km (25 miles) south of the border with Sudan.

Two Sudan armybrigades had also been advancing towards its oil fields in Unity state, backed by militias loyal to Khartoum, he said.

The BBC's James Copnall in Sudan's capital, Khartoum, says the violence will further poison an already very poor relationship.

A presidential summit, which was to have been held in South Sudan's capital, Juba, at the beginning of April, has been postponed indefinitely because of the recent fighting.

African Union mediator Thabo Mbeki held talks late last week with South Sudan's President Salva Kiir and his Sudanese counterpart, Omar al-Bashir, amid international fears of a return to full-blown conflict.