By Michelle Nichols
UNITED NATIONS (Reuters) - U.N. chief Ban Ki-moon on Thursday termed South Sudan's seizure of a disputed oil field in Sudan an "illegal act" and called on both countries to stop border clashes spiraling into war as the United States warned of a "worrying" escalation in rhetoric.
Clashes along the ill-defined border between the former civil-war foes has led to a standoff over the Heglig oil field after it was seized last week by troops from South Sudan, which declared independence last year.
"I call on South Sudan to immediately withdraw forces from Heglig. This is an infringement on the sovereignty of Sudan and a clearly illegal act," Ban, the U.N. secretary-general, told reporters.
"I also call on the government of Sudan to immediately stop shelling and bombing South Sudanese territory and withdraw its forces from disputed territories," he said. "I have impressed on both governments the necessity of ending the fighting and returning to negotiations. They have yet to heed our call."
Mounting violence since Sudan split into two countries last year has raised the prospect of two sovereign African states waging war against each other openly for the first time since Ethiopia fought newly independent Eritrea in 1998-2000.
Sudan's President Omar Hassan al-Bashir all but declared war against his neighbor on Thursday, vowing to teach South Sudan a "final lesson by force" after it occupied Heglig, while Juba accused Bashir of planning "genocide.
"The escalation of rhetoric on both sides is indeed worrying and it's only fanning the flames," U.S. Ambassador to the United Nations Susan Rice, the president of the U.N. Security Council for April, told reporters.
"The effort ought to be made to reduce the flames, douse them, and return to the table to resolve the outstanding issues that have made relations between north and south so difficult in the wake of independence," she said.
On Tuesday, the 15-nation U.N. Security Council discussed the possibility of imposing sanctions on Sudan and South Sudan if the fighting does not stop.
Distrust runs deep between the neighbors, who are at loggerheads over the position of their border, how much the landlocked south should pay to transport its oil through Sudan, and the division of national debt, among other issues.
Both are poor countries - South Sudan is one of the poorest in the world - and the dispute between them has already halted nearly all the oil production that underpins both economies.
South Sudan has accused Sudan of launching air strikes on some of its major oilfields. Sudan has denied launching air strikes but said its ground forces had attacked southern artillery positions that had fired on the north.
South Sudan has said it would only withdraw from the Heglig oil field if the United Nations deploys a neutral force there.
South Sudan seceded from Sudan in July, six months after a referendum agreed under a 2005 peace deal that ended decades of civil war that killed more than 2 million people.
(Additional reporting by Louis Charbonneau; Editing by Vicki Allen and David Brunnstrom)