Sudanese President Omar al-Bashir had vowed to "never give up" the disputed region, which his nation also claims.

CNN: STORY HIGHLIGHTS

Sudanese President Omar al-Bashir had vowed to "never give up" the disputed region.

CNN: South Sudan accuses Sudan of aerial, ground attacks

Sudan claims Heglig's oil fields, which account for about half of the nation's oil production, and Sudanese President Omar al-Bashir had vowed to "never give up" the disputed region.

CNN: South Sudan says it is withdrawing from disputed oil region