President Barack Obama on Wednesday called for an immediate ceasefire in Sudan's South Kordofan, where he said the situation has turned "dire" as militias battle government troops accused of ethnic cleansing.

Heavy fighting has raged since June 5 between the northern army and its allied militias fighting forces aligned with the southern former rebel army the Sudan People's Liberation Army (SPLA), ahead of the south's proclamation of full independence next month.

With the fighting potentially overshadowing the historic event, particularly if the southern army is drawn in, Obama reiterated his call for Sudan's warring factions to end the bloody violence threatening a peace agreement.

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

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 leader said.

While hailing a recent agreement between the government and the Sudan People's Liberation Movement (SPLM) -- the SPLA's political wing -- aimed at reducing tensions and allowing UN peacekeepers into the region, Obama urged the parties to "build on that progress and agree to an immediate ceasefire in Southern Kordofan."

Obama condemned the violence, "in particular the Sudanese Armed Forces' aerial bombardment of civilians and harassment and intimidation of UN peacekeepers.

"With a ceasefire in Southern Kordofan, alongside the agreement to deploy peacekeepers to Abyei, we can get the peace process back on track," he said.

"But without these actions, the roadmap for better relations with the government of Sudan cannot be carried forward, which will only deepen Sudan’s isolation in the international community."

The United Nations on Wednesday described the treatment of civilians, including reported human rights abuses the targeting of people along ethnic lines, as "reprehensible," adding that insecurity and movement restrictions continued to limit the UN's ability to provide emergency aid to some 70,000 people who have fled the fighting.