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.
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".
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.