The European Union has called for an immediate end to fighting in two Sudanese states bordering South Sudan, and urged unhindered access for relief workers.

Europe's voice adds to growing international concern over the situation in South Kordofan and Blue Nile, where war broke out last year and the government has severely restricted foreign relief agencies.

More than 500,000 people have been displaced or severely affected by the fighting, the United Nations estimates.

"The EU is gravely concerned about the ongoing violence in Southern Kordofan and Blue Nile states. It calls for an immediate cessation of fighting," said a statement issued after a meeting of the bloc's foreign affairs council on Monday.

"The EU urges the government of Sudan to allow safe and unhindered access for international humanitarian actors to all civilians and reiterates its readiness to provide humanitarian assistance to all those in need."

Warplanes on Monday dropped several bombs in two attacks targeting a South Sudan transit centre for refugees fleeing the fighting in Blue Nile, the UN refugee agency said.

One boy was wounded and 14 were missing, it said.

Khartoum has cited security concerns in severely restricting foreign aid organisations in South Kordofan and Blue Nile, where fighting erupted several months ago between the Sudan Armed Forces and ethnic minority insurgents once allied to the former rebels who now rule South Sudan.

The South gained independence from Khartoum last July after decades of civil war.

Food shortages would become critical without substantial aid deliveries into the two states by March, the US ambassador to the United Nations, Susan Rice, said last week.

She accused the Khartoum government of "a deliberate policy" of blocking aid agencies from reaching civilians in the conflict zones.

Sudan's UN ambassador accused aid workers of using UN flights to deliver arms and ammunition to the rebels -- a claim for which the UN's top humanitarian official said there was "no evidence."