Mukesh Kapila, an official at rights group Aegis Trust, told the BBC that Sudan was bombing targets in the area indiscriminately.
"We saw water wells being targeted. We saw churches destroyed where people might take shelter from the fighting," he said. "And we saw, for the first time reported, the use of anti-personnel land mines and cluster bombs."
The U.N. Security Council, in debates this week, pressed Sudan and South Sudan to avoid taking further action that would escalate into regional conflicts.
Both sides to the conflict blame the other for stoking unrest.
South Sudan became an independent nation as part of a comprehensive peace agreement reached in 2005, though border conflicts and disputes over oil threaten to continue to unravel the peace.
Khartoum denies the claims. The government added that it wasn't keen on allowing humanitarian agencies into the region that it claims were working to destabilize the situation.
During the Security Council debate, Sudanese officials said the country was being dragged into war.
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