* U.S. says world must take action as famine looms
* Aid groups say Khartoum's stance limits options
* U.S. fears Sudan oil dispute risks long-term damage
By Andrew Quinn
WASHINGTON, Jan 24 (Reuters) - The United States fears a large-scale famine in Sudan's restive border states of Southern Kordofan and Blue Nile and is boosting pressure in Khartoum to accept aid or face a unilateral assistance operation, a senior U.S. official said on Tuesday.
"We are feeling a lot of pressure, if there's no international access, to look at ways in which assistance would be carried across the border without their approval," Princeton Lyman, the Obama administration's special envoy for Sudan, told reporters.
Lyman cited expert reports which said that the worsening crisis, which has already caused tens of thousands of people to flee their homes, could expand to leave more than a quarter of a million people on the brink of famine by March.
"This could be a major, major calamity," he said.
Sudan said this week it would continue to allow only limited access to United Nations agencies and aid groups in the two border states, where government forces have battled rebels since June last year shortly before South Sudan declared independence under a 2005 peace deal..
Both states border South Sudan and are home to tens of thousands of fighters who fought Khartoum as part of the southern army during the civil war. Sudan accuses Juba of continuing to back the insurgents, which South Sudan denies.
Deputy Secretary of State Bill Burns will visit South Sudan as part of an Africa tour next week before heading to Ethiopia for the Jan. 29-30 summit of the African Union, where he is expected to press for a strong regional response to Sudan's emerging food crisis, U.S. officials said.
Lyman emphasized that no decision on U.S. food aid had yet been made, and said any attempt to deliver aid without Khartoum's approval would be fraught with risk.
"We know that the government would be opposed to it," Lyman said. "If you want to do something by March considering positioning of food, etc, it takes quite a while .... so if the (AU) meeting doesn't resolve this by the end of January we're going to be in a serious situation."
Aid experts said the United States had limited options if Khartoum maintains its ban on aid, and voiced concern that existing aid operations in the troubled western region of Darfur could be targeted for reprisals.
"Air drops are not really viable because the cost versus scale is really high," one aid group analyst, speaking on background due to the sensitivity of the situation.
"It could be that they have some idea of what they want to do and are just not sharing it, but certainly the operational NGOs do not have a clear picture of what the plan is."
"OVER THE BRINK" ON OIL DISPUTE
Lyman said Washington was also pressing hard for Khartoum to resolve an oil dispute with South Sudan which has seen the new government in Juba start to shut down oil production in a row over revenues..
The new landlocked nation needs to use a northern pipeline and the port of Port Sudan to export its crude but has failed to reach an agreement with Khartoum over a transit fee, prompting Sudan to start seizing oil as compensation.
Officials said in November South Sudan was producing about 350,000 barrels of oil per day.
China is the biggest buyer of oil from the two countries, purchasing some 13 million barrels last year. That amounted to 5 percent of last year's crude imports by China, which is also the top investor in South Sudan's oilfields.
Lyman said mediators led by former South African President Thabo Mbeki were expected to present a new proposal to negotiators in Addis Ababa on Tuesday which he described as a last-ditch effort to resolve a dispute that threatens to wreak economic havoc on both sides.
"This is a very bad situation and both sides could get hurt very, very badly," Lyman said, noting some assessments which say that a pumping pause of a few days could make it extremely difficult and costly to resume production.
"I can understand the anger. I can understand the response. But I'm very worried that they're going over the brink here," he said. (Reporting By Andrew Quinn; Editing by Eric Walsh)