Northern Sudan's seizure of a contested border region is an act of war, a spokesman for the Southern Sudanese army said Sunday.

Southern forces have completely withdrawn from the town of Abyei after northern troops occupied the town with tanks on Saturday night, said army spokesman Col. Philip Aguer. Both sides claim the fertile region, which is near several important oil fields.

"We didn't declare war," said Aguer. "The (Sudanese ruling party) National Congress Party and the Sudan Armed Forces declared war on us."

Aguer said the southern army stationed in Abyei was overrun and scattered after the north had conducted two days of aerial bombardments, focusing on a bridge where southern reinforcements would have entered. The southern army "will maintain the status quo," he said, while it waits for the decision of the southern government on how best to respond.

Aguer called for the United Nations Mission in Sudan "to protect the people of Abyei," saying that the northern government intends to "displace civilians and commit human rights violations as they did in Darfur."

Several members of the Abyei government were missing, he said.

Aid group Doctors Without Borders said in a statement the entire population of Abyei fled Saturday morning after the bombing raids, gunfights and mortaring. One mortar exploded in a U.N. compound but there were no casualties.

The massive escalation in the most volatile spot along Sudan's contested north-south border came as the United Nations Security Council began a four-day visit to Sudan. The Council's scheduled visit to Abyei has been canceled because of the violence but they are due to make a public statement later on Sunday.

Southern Sudan fought the north for more than two decades in a war that cost around 2 million lives. A peace deal in 2005 offered it the chance for independence and it overwhelmingly voted to secede in a January referendum. It is due to become the world's newest country in less than two months.