* Number is up sharply from earlier estimates
* U.N. says unable to get aid through to needy
KHARTOUM, Oct 19 (Reuters) - More than 900,000 people have been displaced or severely affected by fighting in two Sudanese border states, the United Nations said on Friday, sharply increasing its estimates and urging Sudan and rebels to let in badly needed aid.
Fighting between Sudan's army and SPLM-North rebels broke out in the oil-producing state of South Kordofan in June 2011, shortly before South Sudan became independent.
The violence spread in September 2011 to nearby Blue Nile state which also borders the new African republic.
Sudan accuses South Sudan of backing the Sudan People's Liberation Movement-North (SPLM-North), charges diplomats find credible despite denials by South Sudan.
Altogether 908,000 people have been displaced or severely affected in both states, the U.N. Office for the Coordination of Humanitarian Affairs (OCHA) said, revising upward its previous count of 655,000.
Around 350,000 people were affected in rebel-held areas in South Kordofan and 70,000 in Blue Nile, the U.N. report said.
"In SPLM-N areas, no humanitarian staff have been able to enter from within Sudan and no food aid has been delivered, despite intensive negotiations that have been going on now for more than 16 months," the United Nations said.
In August, Sudan agreed to let aid pass through its territory into rebel-held areas under an agreement brokered by the United Nations, the African Union and Arab League.
But the U.N. has been unable to win approval from Sudan to distribute aid during almost three months of talks. Sudan has blamed the rebels for the delay and state media has criticised the U.N. for failing to attend some meetings to discuss technical details, charges denied by U.N. officials in Sudan.
SPLM-North, which accuses the government of marginalising large parts of South Kordofan and other border areas, has formed an alliance with other rebel groups to try and topple Sudan's veteran President Omar Hassan al-Bashir.
South Sudan declared independence from Sudan in July last year, under the terms of a 2005 peace deal that ended their civil war.
They agreed to set up a buffer zone along their shared boundary last month after coming close to war in April. But there has been scant progress in parallel indirect talks between Khartoum and SPLM-North, which fought as part of the southern rebel army during the civil war. (Reporting by Ulf Laessing; Editing by Michael Roddy)