The United Nations chief says South Sudan's seizure of the Heglig oil fields is illegal and violates the sovereignty of neighboring Sudan.

Speaking to reporters in New York Thursday, Secretary-General Ban Ki-moon asked South Sudan to remove its forces from the oil-rich border area, which is claimed by both countries.

“I call on South Sudan to immediately withdraw its forces from Heglig. This is an infringement on the sovereignty of Sudan and a clearly illegal act.”

Mr. Ban also called on Sudan to stop shelling and bombing South Sudanese territory and withdraw from the disputed Abyei region.

He renewed the U.N.'s call for the two Sudans to stop fighting and return to negotiations to settle their differences.

Earlier, Sudan's President Omar al-Bashir threatened full-scale war against South Sudan, as fighting continued along the two countries' border.

On Wednesday, President Bashir vowed to crush South Sudan's government, likening the south's ruling SPLM party to an insect.

Addressing another rally Thursday, he said Sudan will teach the south's government “a lesson by force,” and vowed to retake Heglig, which southern forces occupied last week.

The two Sudans have been unable to resolve disputes over borders, oil, and citizenship issues stemming from the south's independence last July.

In January, South Sudan shut down all oil production because of a dispute over transit fees to use the north's pipeline and port.

Starting in the 1980s, north and south Sudan fought a 21-year civil war that eventually led to southern autonomy and independence.