Salva Kiir, the South Sudanese president, has said his northern neighbour Sudan has "declared war" on his country, as fighter jets from the north reportedly launched more strikes overnight in a border region.
Although there has yet to be a formal declaration of war by either of the Sudans, Kiir's comments, made on Tuesday during talks on a visit to China, will likely stoke tensions between the rival nations.
While meeting his Chinese counterpart, Hu Jintao, the South Sudanese leader said the visit "comes at a very critical moment for the Republic of South Sudan because our neighbour in Khartoum has declared war on the Republic of South Sudan".
A spokesman for the South Sudan military said Sudanese Antonov airplanes dropped eight bombs between 11pm and 1am in Panakuac, where he said ground fighting had been ongoing since Sunday.
Colonel Philip Aguer said he had not received information on casualties from the attack because of poor communication.
The bombings followed Monday's aerial attacks that targeted a market and an oil field in South Sudan, leaving at least two people dead.
The assault took place after Sudanese ground forces had reportedly crossed into South Sudan with tanks and artillery.
China urges restraint
China has called for an end to weeks of border fighting that saw the South seize Sudan's most important oilfield in the Heglig area on April 10 for 10 days.
Beijing has been a key ally and the largest economic partner of diplomatically isolated Sudan.
But it is keen not to alienate the South, the world's newest nation and a source of oil.
"I think China would be more aware than any other actor that things are on a knife edge at the moment," Larry Attree, a specialist in conflict prevention with the NGO Saferworld, told Al Jazeera from London.
"China has a strong interest in seeing a peaceful outcome between South Sudan and Sudan and it gets 60 per cent of its oil from the combination of the two countries ... and both countries see it as a key partner.
"It's been Sudan's leading trade partner for the last decade or so, and also has a seat on the UN Security Council. In terms of clout, it is the key external actor with the influence on the two parties."
South Sudan said it withdrew its forces from the disputed oil town, but the North claimed its soldiers drove them out.
After heavy fighting that broke out last month, both Sudan and South Sudan are reportedly reinforcing troop numbers and digging into trenches along their contested border.
Taban Deng, the governor of Unity State, said that at present "with the exception of aerial bombardment, the front line is quiet".
|In-depth coverage of North-South strife over border
However, Mac Paul, the South's deputy director of military intelligence, said he had "information from our sources the Sudanese army is mobilising for a push on Bentiu".
His claims, however, could not be verified from independent sources.
Bentiu lies at least 60km from the frontline with Sudan's army, and large numbers of Southern troops and tanks have moved into the border zone to bolster defences.
Sudan denies responsibility, but the president has promised to press ahead with a military campaign until all southern troops or affiliated forces leave territories in Sudan.
Amid the escalating tension between the the two neighbours, Ban Ki-moon, the UN secretary-general, condemned air raids by Sudan, and called on the countries' leaders to return to dialogue.
"The secretary-general condemns the aerial bombardment on South Sudan by Sudanese armed forces and calls on the government of Sudan to cease all hostilities immediately," Eduardo del Buey, deputy UN spokesman, said on Monday.
'Return to dialogue'
"He [Ban] calls on President [Omar] al-Bashir and President [Salwa] Kiir to stop the slide toward further confrontation and urges both sides to return to dialogue as a matter of urgency."
The African Union (AU) has given the rival neighbours two weeks to resume negotiations and three months to settle their disuptes.
Talks between the two countries over the unresolved disputes, that were being mediated by an AU panel led by former South African president Thabo Mbeki, broke down in Ethiopia earlier this month.
AU commissioner Ramtane Lamamra said that if the two parties fail to resolve their disputes in the given time frame, the panel would be disbanded and outstanding issues would then be settled by an unspecified arbitration process.
Bashir, who visited the oil-rich town of Heglig on Monday, said the time for talks with Kiir's government was over.
"No negotiation with those people, our talks with them were with guns and bullets," Bashir told soldiers in the town, which the South occupied for 10 days.
The violence in Heglig is the worst since South Sudan won independence in July last year after a 1983 to 2005 civil war in which an estimated two million people died.
Sudan and South Sudan have been drawing closer to a full-scale war in recent months over the unresolved issues of sharing oil revenues and the disputed border.