A British aid agency has withdrawn nearly two dozen workers from the Upper Nile area of South Sudan following several days of increased violence.

Oxfam says its workers reported hearing bombings and heavy artillery fire Friday near the Sudan border.

A U.N. official said Sudan was responsible for Thursday's bombing near a refugee camp in neighboring South Sudan. There were unconfirmed reports of casualties. Sudan has denied bombing South Sudan's territory.

In response to the violence, Oxfam said Saturday 22 engineers and health workers responsible for providing clean water and sanitation to more than 60,000 people have been relocated away from the volatile border area.

The British aid agency says refugees fleeing violence in Sudan's Blue Nile area will also be impacted by the relocation of the aid workers. Oxfam says it is the sole provider of clean water in for people displaced by the violence.

The aid agency has called for an end to the fighting to protect the civilians and ensure the safety of aid workers.

Meanwhile, a watchdog group says Sudan is expanding its capacity to conduct air strikes along the border with South Sudan.

The Satellite Sentinel Project says images from space show Sudan upgrading air bases it recently captured from rebels in Blue Nile state. It says the upgrades include newly-improved airstrips and new helicopter pads.

Sudan and South Sudan have been almost constantly at odds since the south became independent in July.

Sudan has recognized the south's independence but the sides have yet to resolve issues over borders – especially in the oil-rich Abyei region – and the sharing of oil revenue. The south took over most of Sudan's oil, but pipelines to the sea run through the north.