Sudanese planes bombarded a disputed oil town near South Sudan's border, a southern military official said, and a doctor said bombs aimed at strategic sites in the south's Unity State killed five people.
Col Philip Aguer said villages near the disputed town of Heglig fell under heavy aerial bombardment and he expected ground fighting to resume "anytime" soon.
Troops from South Sudan captured the oil-rich border town that is claimed by Sudan on Wednesday.
Col Aguer said southern forces did not plan to give up the town, which lies along the ill-defined border between the two Sudans.
"As we speak now, the SPLA is still in full control of Heglig," he said, using the acronym for the southern army.
"There are many villages that are being bombed. The Sudanese armed forces are conducting indiscriminate air bombardment by Antonov jet fighters, indiscriminate bombing of Heglig, bombing of oil instalments. They are bombing SPLA positions by long-range artillery."
Col Aguer could not say how many soldiers had been killed on either side. He said at least 19 South Sudanese soldiers and 240 Sudanese troops had been killed since the recent resumption of hostilities.
Heglig has been the focal point of more than two weeks of clashes between the two nations. Both sides claim the area, but Sudan operates Heglig's oil facilities, which account for nearly half of the country's daily production. The town is 60 miles east of the disputed region of Abyei, whose fate was left unresolved when South Sudan split last year from Sudan.
The United Nations Security Council demanded the withdrawal of South Sudan's military forces from Heglig and an end to aerial bombing by Sudan of South Sudan. It also urged Sudanese president Omar al-Bashir and South Sudan's president Salva Kiir to hold a summit to resolve their conflicts.
In Unity State, Bentiu Hospital medical director Peter Gatkuoth said bombs killed four civilians and a soldier yesterday. Jets released six bombs. They targeted a bridge that links Bentiu to a road into Sudan.
Fighting erupted in between Sudan and South Sudan last May, just months before South Sudan formally declared independence from Sudan.
The region was to hold a referendum in January to decide whether it stays with Sudan or joins a newly independent south. But the vote was postponed indefinitely amid disagreements over who would be eligible to vote.
The fighting has displaced more than 100,000 people, most of whom are still waiting to return.
The continued clashes have dimmed hopes for a resolution between the two countries on a host of issues left over from their July split, including oil-sharing, citizenship issues and the demarcation of the border.