The UN has accused Sudan of bombing Elfoj refugee camp in the South Sudan's Upper Nile state, an attack that has heightened tensions between the two countries.

After the Monday attack, 14 civilians have gone missing and several others injured.

According to the UN High Commissioner for Refugees, the aerial bombing was carried out in two instances with several bombs falling at the refugee transit site located less than 10 kilometres from the border with Sudan. About 5,000 refugees were at the site at the time of the bombing.

"UNHCR is alarmed by this attack on vulnerable refugees already fleeing violence in Sudan's Blue Nile state," the UN official added.

The agency has so far relocated 11,477 refugees to safer sites, some 70 kilometres away from the border since January.

Recently, South Sudan announced plans to shut down the pipeline that runs through the Sudan to the export terminal at Port Sudan in response to alleged continuous oil theft by Khartoum.

Last November, New Gufa, an entry point for refugees in Maban County in Upper Nile state, was bombed for several days, leading to several deaths. Sudan claimed that the camps hosted rebels. Yida refugee camp in Unity state, near the border with Sudan's South Kordofan state was also hit by air raids.

At least 78,000 civilians have been displaced by fighting in Blue Nile and Southern Kordofan states, most of whom are camping in Unity and Upper Nile states in South Sudan.