Aid group Oxfam said on Sunday it was pulling out 22 staff from South Sudan's border region to the north after fighting was reported in the area.
The poorly-marked border area has been scene to violence between Sudan and South Sudan since Juba seceded in July under a 2005 peace deal ending decades of civil war with the north.
South Sudan and the United Nations have accused Sudan of having bombed a refugee camp in Unity state on Thursday. Juba also says Khartoum orchestrated an attack in neighbouring Upper Nile state the same day. Sudan denies both charges.
Oxfam said it was removing 22 staff from Upper Nile where the aid group had been providing clean water for 64,000 people.
Oxfam staff there reported bombing and heavy artillery for several hours on Friday, Oxfam said in a statement. They have witnessed planes flying overhead and a build-up of South Sudanese troops over the past few days.
According to South Sudan's army, five soldiers were killed and 26 were wounded when southern mercenaries backed and armed by Khartoum launched an attack in Upper Nile state.
Fighting has erupted this year in Sudan's Blue Nile and South Kordofan states between Sudan's military and rebels who sided with the south during the civil war. Tens of thousands have fled over the border to South Sudan.
Oxfam teams were about to start an assessment in Upper Nile to provide aid to new refugees, Oxfam said. They have fled attacks and walked for days to reach a place they thought would be safe.
(Reporting by Ulf Laessing)