Sudan and newly-independent South Sudan accused each other of launching fresh attacks on their territories on Sunday as neither side showed any sign of bowing to global pressure to return to the negotiating table.
South Sudan said Sudanese troops attacked settlements about 10km (6 miles) on its side of the border and carried out air raids in a range of areas including its oil-producing Unity state.
We are building up troops because we think that the Sudanese army is also building up, Mac Paul, deputy director of South Sudan's military intelligence, told reporters in the southern border town of Bentiu.
Sudan denied the accusations but said it had repelled a major attack by SPLM-N rebels in South Kordofan state, on its own side of the border. Sudan routinely says the rebels are controlled by the South.
Tensions have mounted since South Sudan declared independence from Sudan in July last year, under a peace settlement that ended decades of civil war between the two sides.
In the worst fighting since the split, South Sudan earlier this month seized the disputed oil-producing territory of Heglig, raising fears of a return to all-out war - then announced it had started withdrawing on Friday, following sharp criticism from U.N. Secretary-General Ban Ki-moon.
U.S. President Barack Obama urged the leaders on Saturday to restart stalled negotiations over a series of territorial and oil-related disputes, saying there was still a chance to avoid war.
But there was no letup in the bellicose rhetoric over the weekend.
The South's Paul described the Sudanese attacks as a serious invasion of our territory.
Sudan's State Oil Minister Ishaq Adam Gamaa told Reuters the chance of the sides reaching a settlement soon was now very remote and said Khartoum would probably demand compensation for damage to Heglig before returning to talks.
Sudan lost about 40,000 barrels per day of output because of the fighting, he said, but added the country had enough reserves to last up to six months before the impact would be felt in its refineries.
Any return to widespread fighting would have a devastating impact on both oil-dependent countries and push refugees and fighters into the surrounding region.
The fighting has already shut down most of the oil production that fuels both Sudan and South Sudan's struggling economies.
In the Sudanese capital Khartoum, hundreds of Muslims stormed a Christian church complex used by southerners in Khartoum on Saturday, witnesses said, raising fears that the border fighting was also stoking ethnic tensions in the city.
Sudan is overwhelmingly Muslim while most southerners follow Christian and traditional African beliefs. Hundreds of thousands of southerners, and people from regions close to the shared border, have remained in Khartoum, many of them in a state of legal limbo.
On Sunday, South Sudanese officials showed reporters an oil field in Unity which they said had been bombed by the Sudanese air force last week.
A Reuters reporter saw three bomb craters at an oil field run by the Greater Nile Petroleum Operating Co (GNPOC) but no visible damage to facilities.
A satellite monitoring group also on Sunday said images showed an oil collection manifold in Heglig appeared to have been destroyed in the fighting.
Limited access to the remote border conflict areas makes it difficult to verify the often contradictory statements from both sides.
South Sudan won its independence in a referendum that was promised in a 2005 peace accord that ended decades of civil war between Khartoum and the south. Religion, ethnicity and oil fuelled that conflict, which killed about 2 million people.
(Additional reporting by Ulf Laessing and Khalid Abdelaziz in Khartoum; Writing by Andrew Heavens; Editing by Michael Roddy)