Washington: US President Barack Obama on Wednesday called for an immediate ceasefire in Sudan's South Kordofan state, where he said the situation is "dire" with Government forces accused of ethnic cleansing.

Heavy fighting in the run-up to south Sudan's declaration of independence on July 9 has pitted Government troops and allied militias against forces aligned with the southern Sudan People's Liberation Army (SPLA).

The fighting threatens to torpedo the US-brokered 2005 Comprehensive Peace Agreement (CPA) that ended a 22-year civil war between forces of the northern-based government of Sudan and the SPLA.

"The situation in Southern Kordofan is dire, with deeply disturbing reports of attacks based on ethnicity," Obama said in a statement.

Both parties must now "end the current violence and allow immediate humanitarian access to desperate people who have been driven from their homes and are now cut off from outside help," the US President said.

Church leaders and activists say the Northern Army's Campaign forms part of a Government policy of ethnic cleansing, targeting the state's indigenous Nuba peoples who fought with the SPLA during the 1983-2005 civil war.

Khartoum strongly rejects allegations of ethnic cleansing and insists it is protecting civilians in South Kordofan, the north's only oil-producing state.