South Sudan's President Salva Kiir says Sudan has "declared war" on his country, following weeks of fighting along their common border, BBC News reported.

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, April 23.

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.