South Sudan accused its northern neighbour Sudan on Tuesday of bombing the disputed border town of Jau, killing four soldiers and breaking a non-aggression pact the two former civil war foes signed last week.
Sudan denied the charges and said it was respecting the agreement.
Relations between the two countries have plunged after talks failed to halt an oil export dispute, end violence in border areas and resolve other issues relating to the South's secession last year under a 2005 peace deal.
The governments signed a non-aggression pact just two days before the reported bombing. The agreement, brokered by the African Union in Addis Ababa, aimed to defuse tensions over the row, which officials have warned could trigger a war.
South Sudan's army spokesman Philip Aguer said a Sudanese Antonov cargo plane dropped several bombs on a military base in Jau, a town straddling the poorly defined border.
"I doubt the non-aggression pact means anything on the ground," Aguer told Reuters, adding four soldiers from the south's Sudan People's Liberation Army were killed in the bombing.
Al-Sawarmi Khalid, Sudan's military spokesman, said the accusation was false.
"We stress absolutely that the Sudanese armed forces did not bomb any area inside South Sudan, and that we are respecting the non-aggression agreement we recently signed," he said. "There is no reason for us to bomb the South, because we are not at war with the South."
Juba and Khartoum routinely trade accusations of sponsoring insurgencies in each other's territory. In December, the two armies clashed in Jau, which is close to many of the South's oil fields and abuts Sudan's restive South Kordofan state.