Sudan vows to retake Heglig oil fields from South Sudan
Sudan has vowed to use "all legitimate means" to repulse South Sudan from its largest oil field, a statement on the official Suna news agency says.
South Sudanese troops seized control of Heglig on Tuesday, as heavy fighting raged for a second day.
Both sides blame each other for the latest conflicts along the undemarcated and disputed oil-rich border area.
Fierce clashes over the past two weeks have fuelled fears of a return to outright war.
Oil-rich Heglig is usually recognised as being part of the north, although South Sudan disputes this.
Khartoum says it will use all legitimate means to repulse what it calls South Sudanese aggression.
"The government of Sudan announces after this attack that it will react by all means," the official statement said, warning of "destruction" in the South.
The statement also warned the new state's "aggressive behaviour" will "bring nothing but ruin and disappointment to its people".
South Sudan's military spokesman Philip Aguer said the south was holding positions in the Heglig oil field.
He said its troops advanced to Heglig after they were attacked by Sudanese troops from the ground and the air.
South Sudan gained independence from Khartoum last July after a long civil war - but the two countries remain at loggerheads.
The BBC's James Copnall in Khartoum says oil is at the heart of the disagreements between the two countries, and oil installations are increasingly being targeted militarily.
In January, South Sudan, which depends on oil sales for 98% of its revenue, shut down all of its oil fields in a row over the fees Sudan demands to transit the oil.
A presidential summit, which was to have been held in South Sudan's capital, Juba, at the beginning of April, has been postponed indefinitely because of the recent fighting.
African Union mediator Thabo Mbeki held talks late last week with South Sudan's President Salva Kiir and his Sudanese counterpart, Omar al-Bashir, amid international fears of a return to full-blown conflict.
Meanwhile, an international weapons monitoring group, Small Arms Survey, says it has gathered enough evidence to show that both South Sudan and Sudan are providing arms to rebels and militia groups in each other's territory.
Both sides have often made and denied such claims of support.