South Sudan's Salva Kiir says Sudan has declared war
South Sudan's President Salva Kiir says Sudan has "declared war" on his country, following weeks of fighting along their common border.
Mr Kiir was speaking in China, which is a major buyer of oil from both countries, but has long been an ally of Sudan's President Omar al-Bashir.
Meanwhile, Sudanese warplanes conducted multiple bombing raids against Southern border regions in the early morning.
The raids followed a fatal bombing near the border town of Bentiu on Monday.
UN Secretary General Ban Ki-moon condemned the attack, in which a market was bombed, killing at least one person and injuring many others.
The latest attacks hit the towns of Panakwatch and Lalop, and the Teshwin border post, the AFP news agency reported.
South Sudan became independent last year, following decades of conflict.
There have been tense relations since then, primarily over the division of oil reserves and the full definition of borders.'Strategic partner'
In the bombing of Bentiu on Monday, we could hear planes overhead, a number of explosions. One of the principal targets was the bridge, though it was the market where at least one civilian was killed.
And as soon as those planes flew overhead, South Sudanese soldiers, though also civilians, started opening fire with small arms. They had relatively little chance of hitting those planes; it might have been an emotional thing almost to try to strike back.
And then last night in Bentiu town, there was an awful lot of gunfire; it is not quite clear what it was about, whether people were shooting at imaginary planes in the sky, or possibly looting.
There are suggestions that soldiers are upset with the way things are going. One, out of the corner of his mouth, said: "A lot of people are dying" as he walked past us on the road.
Mr Kiir was speaking as he met Chinese counterpart Hu Jintao in Beijing, after arriving there on Monday for a five-day visit.
Mr Kiir said his visit came "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".
He called China one of his country's "economic and strategic partners".
Chinese state television quoted Mr Hu as urging calm and restraint on both Sudans.
Sudan has made no formal declaration of war, but analysts say Mr Kiir is clearly escalating the war of words.
Beijing has urged an end to the recent hostilities, during which Southern forces occupied Sudan's most important oil field, in the Heglig area, saying it belonged to the South.
South Sudan says its forces withdrew from Heglig after two weeks, but Sudan says it expelled them, killing 1,000 soldiers.
Mr Bashir says he will not negotiate with the South and has vowed to continue military action until all Southern troops and their allies are out of Sudan.
The BBC's James Copnall, recently in Bentiu, says that while the conflict has not spread beyond a limited geographical area, talk of a slide towards war is not far from the truth.
On Monday, Mr Ban called on Mr Bashir and Mr Kiir "to stop the slide toward further confrontation and... to return to dialogue as a matter of urgency".
Main disputes between the two Sudans
- Transit fees the South should pay Sudan to use its oil pipelines
- Demarcating the border
- Both sides claim Abyei
- The rights of each other's citizens now in a foreign country - there are estimated to be 500,000 southerners in Sudan and 80,000 Sudanese in the South
- Each accuses the other of supporting rebel groups on its territory
US President Barack Obama has said both countries "must have the courage" to return to the negotiating table and resolve their differences peacefully.
China's foreign ministry spokesman Liu Weimin said on Tuesday that oil was "the economic lifeline for both countries".
He added: "To maintain the stability and sustainability of the oil cooperation is consistent with the fundamental interests of both countries. It is also consistent with the interests of Chinese enterprises and their partners.
"We hope the oil negotiation between Sudan and South Sudan will make progress and [the two countries] will find a solution that both of them and other sides involved can accept."
In January, South Sudan shut down oil production, which provides 98% of its revenue, after Khartoum impounded South Sudanese oil shipments amid a dispute over transit fees.
South Sudan took most of the former united Sudan's oil reserves when it became independent but relies on pipelines to seaports in Sudan to export it.
South Sudan voted overwhelmingly in favour of secession in a January 2011 referendum, leading to independence six months later.
Both Sudan and the South are reliant on their oil revenues, which account for 98% of South Sudan's budget. But the two countries cannot agree how to divide the oil wealth of the former united state. Some 75% of the oil lies in the South but all the pipelines run north. It is feared that disputes over oil could lead the two neighbours to return to war.
Although they were united for many years, the two Sudans were always very different. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
The health inequalities in Sudan are illustrated by infant mortality rates. In South Sudan, one in 10 children die before their first birthday. Whereas in the more developed northern states, such as Gezira and White Nile, half of those children would be expected to survive.
The gulf in water resources between north and south is stark. In Khartoum, River Nile, and Gezira states, two-thirds of people have access to piped drinking water and pit latrines. In the south, boreholes and unprotected wells are the main drinking sources. More than 80% of southerners have no toilet facilities whatsoever.
Throughout the two Sudans, access to primary school education is strongly linked to household earnings. In the poorest parts of the south, less than 1% of children finish primary school. Whereas in the wealthier north, up to 50% of children complete primary level education.
Conflict and poverty are the main causes of food insecurity in both countries. In Sudan, many of the residents of war-affected Darfur and the border states of Blue Nile and South Kordofan, depend on food aid. The UN said about 2.8m people in South Sudan would require food aid in 2013. The northern states tend to be wealthier, more urbanised and less reliant on agriculture.