Adisplaced family carrying their belongings walks in search of refuge towards the village of Aburoc, South Sudan, last week. A new report by Amnesty International says South Sudanese forces burned, shelled and ransacked homes between January and May, killing civilians and forcing thousands from the Shilluk ethnic minority to flee. (Photo: Sam Mednick/AP)

South Sudan's government says it may withhold permission for aid workers to go to some rebel-held areas on security grounds, the president's spokesman said on Thursday, after the UN complained aid convoys were being blocked.

"We cannot allow them (aid workers) to go and then be hit by wrong elements against peace and then the government will be blamed," said the spokesman, Ateny Wek Ateny.

"It is about safety for the humanitarians. It is to be coordinated, and the government can only clear the humanitarian workers when it is safe."

South Sudan's four-year-old civil war has killed tens of thousands of people and displaced more than three million. Earlier this year, the UN briefly declared a famine in the northern rebel stronghold of Unity state.

Ateny's comments follow UN complaints that the government has blocked aid deliveries to rebel-held areas in the southern Equatoria region over the last two months, but allowed aid to reach government-held towns.

Since May, four aid convoys were prevented from reaching 30 000 displaced civilians in rebel-held areas in Central Equatoria state's Kajo Keji county, the UN Office for Coordination of Humanitarian Affairs (Ocha) said in a statement on Wednesday.

Authorities also restricted relief groups from rural areas around Torit town in Eastern Equatoria state, Ocha said.

Both counties face "emergency" levels of food insecurity, one step below famine, according this month's report from the Integrated Food Security Phase Classification system (IPC), a government-led statistical body.

In Western Equatoria, aid groups were denied access to the rebel-held villages Kotobi and Bangolo, the UN said.

But aid deliveries have reached government-controlled towns, Ocha noted. South Sudan's government holds most major towns in Equatoria and the rebels are in the countryside. Aid groups are mostly based in the government-controlled capital, Juba.

Kenyi Erastus Michael, the rebel-appointed commissioner in Kajo Keji county, urged aid groups to send relief from neighboring Uganda, which borders rebel-held areas, instead of trying to send convoys across front lines from Juba.

"Humanitarian agencies … want to bring their things from their main stores in Juba with permission from the government, and then it is the government sabotaging it," Michael said by phone.

South Sudan's civil war is one of the world's most dangerous for aid workers. At least 79 have been killed since the start of the civil war, including six in a single attack in March.