WASHINGTON - The United States government on Wednesday expressed concerns on the statement by Sudanese President Omar al-Bashir, in which he vowed to topple the government in South Sudan.

"Obviously, given the escalation of violence over the past few weeks, given the rhetoric that's being thrown about, we're very concerned," said State Department spokesman Mark Toner.

Addressing a rally in the capital city of Khartoum, Bashir on Wednesday declared war on South Sudan, and vowed to bring down the government of the Sudan People's Liberation Movement (SPLM) ?in Juba, as military confrontations escalated on the border between the two countries.

"We continue to, as we said, through Princeton on the ground as well as publicly here, call for both sides to get back to the AU process," said Toner, referring to the U.S. special envoy to Sudan Princeton Lyman, who is in Khartoum meeting with Sudanese officials. On Monday, Lyman visited Juba, the capital of South Sudan, and met with South Sudanese President Salva Kiir Mayardit.

Toner reiterated the U.S. call for an "immediate and unconditional cessation of violence" by both parties.

"That means we want to see the immediate withdrawal of South Sudanese forces from Heglig and we want to see the immediate end to all aerial bombardments of South Sudan by the Sudanese armed forces," he said.

The oil-rich Heglig region on the north side of the 1956 border between Sudan and South Sudan, has been captured by the South Sudanese army since last Tuesday.

South Sudanese President Salva Kiir Mayardit said last month that Heglig belonged to his country, but both Khartoum and the African Union denied the claim.

The pan-African body, along with the United Nations, called for an unconditional withdrawal of the South Sudanese troops from Heglig.

The Sudanese parliament decided last Wednesday to announce a general mobilization and stop negotiations with South Sudan.