Ban Ki-moon, the UN secretary-general, has condemned air raids by Sudan on South 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.
Ban "reiterates that there can be no military solution to the disputes between Sudan and South Sudan", the spokesman said.
"No negotiation with those people, our talks with them were with guns and bullets. "
-Omar al-Bashir, Sudan's president
"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 UN chief’s call for calm came after Bashir ruled out any future talks with his southern counterpart, who is in Beijing to drum up support from China.
Kiir was expected to plead his case to Chinese authorities, but analysts said China was unlikely to take sides and would keep pushing for talks.
China 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 source of most of Sudan's oil.
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.
Meanwhile, Jimmy Carter, the former US president, has told Al Jazeera that the international community needs to send in peacekeepers to the disputed region.
"I would say, send an additional element of peace keeping forces which is going to cost some money, but which so far has been opposed by the US and others because we already have three UN organisations there," he said.
"I believe both Bashir and Kiir will go along with ceasefire if they knew that there were international peace keepers along the disputed border."
South Sudan said it withdrew its forces from the disputed oil town, but the North claimed its soldiers drove them out.
On Monday, Sudan launched air strikes on the border town of Bentiu, the capital of the South's Unity state, killing at least two people and injuring three others.
Sudan denied the attack, but the US condemned it and urged an "immediate" halt to hostilities and a return to talks.
The presidents of Sudan and South Sudan "must have the courage to negotiate" because their people deserve peace, Barack Obama, the US president, said.
|In-depth coverage of North-South strife over border
Mac Paul, the South's deputy director of military intelligence, called the attack "a clear provocation".
The violence in Heglig was 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.
The Sudanese military said its army had killed 1,200 South Sudanese troops in fighting in the area, but South Sudan claimed only 19 of its soldiers were killed and that 240 Sudanese troops lost their lives.
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.
Talks between the two countries over the unresolved disputes, that were being mediated by the African Union, broke down in Ethiopia earlier this month.
As the tension rises between the two archfoes, rights body Human Rights Watch has pointed towards the grim situation of civilians in Blue Nile state of Sudan.
Civilians continue to endure Sudan’s indiscriminate bombing and other abuses in the state, the rights body said, based on a research trip this month.