The United Nations says it is “alarmed” by air attacks on a South Sudan refugee camp holding about 5,000 people who had fled violence in neighboring Sudan.

The U.N. refugee agency says Tuesday the raid took place in the Upper Nile state town of Elfoj, about 10 kilometers from the Sudan border.

The agency reports that several bombs fell during two attacks on a refugee transit site on Monday morning. It says the bombings injured one boy and that 14 others are missing.

The agency did not indicate who carried out the strikes. South Sudan has accused Sudan of bombing its territory in the past.

U.N. officials say the bombings targeted refugees who were especially vulnerable because they had just fled violence in Sudan's Blue Nile state.

The United Nations says that since August more than 78,000 people have fled fighting in Sudan's Blue Nile and Southern Kordofan states along the border. Khartoum has been battling rebels it says are aligned with South Sudan, which split from the north in July.

Aid groups say fighting along the border area has moved closer to refugee camps in South Sudan and humanitarian workers are currently working to transfer the refugees farther south.

The U.N. says a massive humanitarian effort is needed to handle the flow of refugees into South Sudan.

The world body has also expressed concern about the humanitarian needs of people who remain in Sudan's Blue Nile and Southern Kordofan states.

Sudan has denied requests by humanitarian groups to assess the needs of people in the troubled states and provide them with aid.

Earlier this month, U.N. humanitarian chief Valerie Amos said there are reports that food insecurity and malnutrition rates in Blue Nile and Southern Kordofan have reached “alarming” levels.