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.

In South Sudan Thursday, the country's chief negotiator, Pagan Amum, said Juba is ready to halt the fighting.

“The government of South Sudan is ready to sign the cessation of hostilities agreement that was presented to the parties by the panel on 4th of April, and we are ready to sign it now.”

Amum went on to say South Sudan wants to ensure both sides withdraw from all contested areas and settle their land disputes through international arbitration.

Sudan's president, Omar al-Bashir, is threatening full-scale war against South Sudan, as fighting continues 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. 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.

African Union mediators have tried to help the sides settle their issues, but made little progress.

In January, South Sudan shut down all oil production because of a dispute over transit fees to use the north's pipeline and port. The move has resulted in financial loss for both sides.

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