JUBA - The United Nations said on Tuesday it had started airdrops to deliver emergency food to a camp in South Sudan packed with people fleeing fighting on the Sudan side of the joint border.

Fighting broke out between the Sudanese army and rebels of the SPLM-North in South Kordofan state around the time of South Sudan's independence in July 2011. It then spread to nearby Blue Nile state in September.

Sudan accuses South Sudan of supporting the SPLM-North, which says it is marginalised in the Arab country. The South denies any links to the rebels, but some Western diplomats say they find Khartoum's claims credible.

The United States and aid groups have warned there is a risk of famine in both states, especially in areas held by rebels.

More than 170,000 people have fled to South Sudan, where they stay in crowded makeshift camps near the border, the U.N. World Food Programme said.

"As we speak, we've started an airdrop operation into Maban (camp). The planes have started flying today from Gambella, Ethiopia, with food dropping into Maban," Chris Nikoi, country head of WFP in South Sudan, told Reuters in Juba.

He said food was running short in the camp due to a new influx of refugees from Blue Nile state, where some 120,000 people have fled since the outbreak of the fighting.

Reaching refugees was difficult as many parts of South Sudan are not accessible by road during the summer rain season.