( 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 .
Oxfam says its workers reported hours of bombing and heavy artillery in South Sudan . The aerial bombardment comes a day after a refugee camp was bombed . The EU foreign policy chief urges the Sudanese Armed Forces to halt its air attacks .