South Sudan's army spokesman has said Sudan has attacked a disputed oil-producing border region with warplanes and artillery.
Philip Aguer said Tuesday's clashes erupted when troops from Sudan entered the Heglig region which they held for sometime before they were driven out.
"They launched a new attack, and occupied southern territory until the SPLA [Sudan People's Liberation Army] repulsed them," said Aguer. "Fighting continued today and is still ongoing."
He said the Sudanese ground forces had started their attack from the disputed area of Heglig, where Sudan controls an oil field that accounts for roughly half of its daily 115,000 barrel output.
Barbana Benjamin, South Sudan government spokesman, told Al Jazeera that four civilians, including a small child, were wounded in the fighting.
He said two brigades from the Sudanese army accompanied by "mujahedeen and other militias" took part in the attack.
Al Jazeera's Hytham Owait, reporting from South Sudan's capital Juba, said: "The SPLA offensive is expected to impact future relations between the two countries, along with the currently stalled negotiations."
The fighting is the the latest flare-up of violence that has delayed a summit between the former civil war foes.
The SPLA said the town of Teshwin in the border area had come under attack late on Monday and that fighting was continuing on Tuesday.
Al-Sawarmi Khalid Saad, Sudan's armed forces spokesman, could not immediately be reached for comment on his mobile phone.
South Sudan, which seceded in July, has been locked in a bitter dispute with Khartoum over oil payments and other issues, and clashes in the ill-defined border region last month raised concerns they might escalate into full-blown war.