(CNN) -- Oxfam will relocate workers and scale back operations in South Sudan's Upper Nile state following a surge of violence along the border with Sudan, the international aid organization said Saturday.
The charity has been providing clean water and sanitation for 64,000 people in the Upper Nile state.
But Oxfam said it feared for its workers' safety after they reported bombing and heavy artillery over the course of several hours Friday.
"They have witnessed planes flying overhead and a build-up of South Sudanese troops over the past few days," Oxfam said in a statement.
The aerial attacks in Upper Nile state follow the bombing Thursday of the Yida refugee camp in South Sudan's oil-rich Unity state, which prompted an international outcry.
Tensions have been rising since South Sudan gained independence from Sudan in July. The Sudanese army says it is fighting rebel groups in the border region.
The organization's relocation of 22 staff, mostly engineers and health workers, from Upper Nile state will mean those who have fled face further hardship, it said. It estimates 50,000 have crossed the border from Sudan's Blue Nile state since August -- and thousands more are still coming.
"They have fled attacks and walked for days to reach a place they thought would be safe -- but instead they are now facing more violence," said Sanjay Awasthi, head of Oxfam in South Sudan. "In desperate need of food, water and shelter, the refugees now receive virtually nothing when they arrive."
EU foreign policy chief Catherine Ashton urged the Sudanese Armed Forces to "put an immediate halt to aerial bombings in the border areas" in a statement Saturday.
Ashton "strongly condemns the aerial bombardment carried out by the Sudanese armed forces in Upper Nile and Unity States in South Sudan," including the attack on the Yida camp, a spokeswoman said. "Such attacks represent a dangerous escalation of the situation and serve to increase tension at a time when Sudan and South Sudan need to reach a negotiated solution on outstanding issues."
The U.N. High Commissioner for Human Rights Navi Pillay called Friday for an "independent, thorough and credible investigation" into the bombing at Yida.
The camp houses about 20,000 refugees who have fled the violence in the states of Southern Kordofan and Blue Nile.
Those two states, along with the Nuba Mountain region, straddle Sudan and South Sudan's ethnic and political lines.
Although these territories are geographically part of Sudan, the population has faced "exclusion, marginalization and discriminatory practices that have resulted in their opposition to the Sudanese government," according to the U.N. human rights office.
The two sides fought a bitter, bloody civil war over decades that cost as many as 2 million lives. Before the independence of South Sudan in July, human rights monitors expressed concerns that long-standing grievances could end in violence consuming the region again.
CNN's Moni Basu reported from Atlanta and journalist Isma'il Kushkush reported from Khartoum, Sudan.