Image caption Tension has been growing along the border

British aid agency Oxfam is withdrawing its staff from a volatile border region of South Sudan in response to increasing instability.

Oxfam said there had been aerial bombing and artillery fire for several hours on Friday in Upper Nile state.

The agency says thousands of refugees are streaming across the border from the north in need of food and water.

South Sudan became independent in July but tension with Sudan appears to be growing along the border.

The United Nations' head of peacekeeping, Herve Ladsous, said it risked becoming a large-scale confrontation between Sudan and South Sudan.

'Military build-up'

An Oxfam spokesman in Nairobi, Alan McDonald, said staff were very close to the bombings in the Upper Nile.

"They could hear the bombings and they've heard the artillery and they've seen the military build-up for the past few days.

"And fortunately none of our staff were injured or caught up in the actual violence but the increased insecurity and the increased military presence over the past few days just means that we have to suspend some of our operations because the area is becoming more and more unsafe."

Among its work in the Upper Nile, the charity drills boreholes to provide steady supplies of clean water and tries to improve the health of local people by preventing the spread of diseases such as malaria.

Meanwhile, UN human rights chief Navi Pillay has called for an investigation into the bombing of a refugee camp in oil-rich Unity state, which is just south of the border.

Sudan, which is fighting pro-southern rebels on its side of the frontier, denied it was responsible for the air raid, which killed 12 people.