(Adds northern denial, U.N. report)
By Hereward Holland
JUBA, Nov 17 (Reuters) - South Sudan accused Sudan on Thursday of supplying artillery to support a cross-border attack by "mercenaries" on its oil-producing Upper Nile state a day earlier, and called on the United Nations to investigate.
Sudan's armed forces dismissed the charge and repeated it did not back rebels and had not launched any attacks on South Sudan's side of the border.
Border violence has raised tension between the old civil war foes, with both nations regularly trading accusations of supporting insurgencies in each other's territory since South Sudan's secession in July.
"These are mercenaries being trained and armed by Khartoum," South Sudan's government spokesman Barnaba Marial Benjamin told Reuters by telephone. "These militias come and they (Khartoum) support them with artillery. They have crossed the border."
The charge that the Sudan Armed Forces (SAF) were actively involved in Wednesday's raid is likely to test relations already strained by the bombing of Yida refugee camp in South Sudan last week that was witnessed by a Reuters reporter.
Khartoum denied responsibility for the air strike.
Philip Aguer, spokesman for the Sudan Peoples' Liberation Army, said three attackers were killed in the assault on the Kuek area before a retreat back across the border. He said two SAF tanks gave support to the raid.
"The tanks were shelling SPLA positions using 100mm mortar," he said by telephone. "This is the first time that their involvement (in the border unrest) has become more obvious."
"The mercenaries are from South Sudan, who are recruited and under the payment of Khartoum."
Sudanese army spokesman Al-Sawarmi Khalid dismissed the accusation. "This information is wrong. We don't support rebels or mercenaries in South Sudan," he told Reuters.
In a separate incident on Wednesday, rebel forces aligned to renegade South Sudanese general George Athor killed nine people, including six civilians, during an attack on Atar in the state of Jonglei, Aguer said.
He said Sudanese Antonov planes also dropped three bombs deep inside South Sudanese territory on Wednesday. No casualties were reported from the air strike on Kino in Raja county in Western Bahr el Ghazal state at 3.30 p.m. (12:30 GMT), he said.
"For all these attacks, including the bombardment, we request the U.N. to come and verify all these violations and the involvement of the government of Sudan in destabilising and causing insecurity in South Sudan."
The United Nations said it struggled to support the 20,000 people residing in the bombed Yida Camp with food because of recent mining in the area.
"Food rations were delivered on 10 November. However, the quantity was only enough to feed the displaced population for one day," the U.N. Office for the Coordination of Humanitarian Affairs said in a report.
South Sudan broke off into a separate country after voting for independence in a January referendum that was promised in a 2005 peace deal that ended decades of civil war.
An estimated 2 million people died in that conflict. (Additional reporting by Ulf Laessing; Editing by Alexander Dziadosz and Mark Heinrich)