South Sudan on Friday accused Sudan of orchestrating fresh cross-border attacks on its troops, the same day the United Nations said Sudanese military aircraft bombed a refugee camp in South Sudan.
Sudan denied both allegations and said it had not engaged in any military operations south of the border.
Sudan and South Sudan have been at loggerheads over violence along the poorly-defined border since the south seceded in July, complicating talks over unresolved issues such as how much South Sudan should pay to use Sudan's oil pipelines and facilities.
Each side says the other is backing rebels in its territory, and analysts say the conflict could erupt into war.
South Sudan's army spokesman Philip Aguer said southern mercenaries supported and heavily armed by Khartoum launched an attack on a military base in Kuek in South Sudan's Upper Nile state on Thursday around 9.00 a.m. (0600 GMT).
Five soldiers from South Sudan's Sudan People's Liberation Army (SPLA) died and 26 were wounded during the assault, Aguer said. He said 13 of the attackers were killed and 47 wounded.
Their (Khartoum's) plan, as we know, is to occupy some oil fields, Aguer said.
Sudan's armed forces spokesman Al-Sawarmi Khalid dismissed the accusation. We have no war or conflict with the government of South Sudan, he said. This information is not correct.
Oil-producing South Sudan split off into Africa's newest nation after voting for independence in a January referendum. The plebiscite was promised in a 2005 peace deal that ended decades of civil war.
But fighting has erupted this year in Sudan's Blue Nile and South Kordofan states between Sudan's military and rebels who sided with the south during the civil war.
At least five bombs fell in the Yida refugee camp in Unity on Thursday, less than 25 kilometres (15 miles) from the border with Sudan. A Reuters correspondent saw one unexploded bomb wedged in a school and a white plane flying north.
About 20,000 people fleeing fighting in South Kordofan had taken refuge in the camp.
The United Nations on Friday accused Sudan of carrying out the bombing and called for an investigation into the attack.
Yesterday UNMISS (the U.N. mission in South Sudan) confirmed that the Sudan Armed Forces dropped at least two bombs near the Yida refugee camp, U.N. peacekeeping chief Herve Ladsous told a meeting of the U.N. Security Council.
Sudanese U.N. ambassador Daffa-Alla Elhag Ali Osman told the council: There have been no bombings, later adding the reports had been concocted by media agencies supporting anti-Khartoum rebels.
U.S. Ambassador Susan Rice reiterated Washington's condemnation of the attack, calling it outrageous. She added that Osman had blatantly lied in his denial to the council.
Speaking to reporters, she also ordered Khartoum to halt all military operations against South Sudan and urged Juba to be restrained and not let itself to be provoked by the north.
Washington is drafting a U.N. Security Council statement condemning the bombing which it hopes the 15-nation body will adopt on Tuesday in New York, council diplomats told Reuters.
Also on Friday, a satellite monitoring group said Sudan's military was improving air bases in Blue Nile, potentially helping it step up air strikes in the border region.
The Satellite Sentinel Project said it captured images that appeared to show active enhancement of two air bases seized from rebels in Kurmuk on November 2, including what appeared to be four new helicopter landing pads built in the past week.
Sudan foreign ministry spokesman El-Obeid Morawah dismissed the report as an attempt to garner support for armed insurgents and said Sudan does not use military aircraft against civilians or in South Sudan's territory.
(Additional reporting by Louis Charbonneau in New York and Tom Miles in Geneva; Writing by Alexander Dziadosz; Editing by Andrew Heavens)