South Sudan's armed forces on Friday accused Khartoum of orchestrating an attack in the south's Upper Nile border state a day earlier that killed 18 troops.
Sudan's army was not immediately available to respond but has previously denied backing rebels in South Sudan and said it has respected international law and its old civil war foe's boundaries.
Five soldiers from South Sudan's Sudan People's Liberation Army (SPLA) died and 26 were wounded during the assault, Sudan's army spokesman Philip Aguer said. He said 13 of the attackers were killed and 47 wounded.
Aguer said southern mercenaries supported and heavily armed by Khartoum launched the attack on Kuek in South Sudan's Upper Nile state on Thursday around 9.00 a.m. (0600 GMT) and that fighting continued until around 4.00 p.m.
This was done under the orders of Khartoum, he said.