(CNN) -- At least two bombs were dropped near the Yida refugee camp in South Sudan, resulting in an undetermined number of casualties, the spokesperson for U.N. Secretary-General Ban Ki-moon said Thursday.
"We are very concerned that these bombs were dropped in an area where there are thousands of refugees who have gathered after fleeing the violence in Southern Kordofan and Blue Nile states," the spokesperson said in a statement.
"It is essential that both parties immediately take all steps to protect civilian lives."
In Washington, the office of the White House press secretary said in a statement that the United States "strongly condemns the aerial bombardment by the Sudan Armed Forces of the town of Yida," where more than 20,000 refugees who have fled conflict in the Sudanese state of Southern Kordofan are living.
The Southern Kordofan, Blue Nile State and Nuba Mountain regions straddle Sudan and South Sudan's geographical and political lines. Although these territories are geographically part of Sudan, its population has faced "exclusion, marginalization and discriminatory practices that have resulted in their opposition to the Sudanese government," according to the U.N. Office of the High Commissioner for Human Rights.
"This bombing of civilians and humanitarian workers is an outrageous act, and those responsible must be held accountable for their actions," the statement said.
The attack follows other bombardments by the Sudan Armed Forces on November 8 near the border that increase the potential for confrontation between Sudan and South Sudan, it said.
"The United States demands the Government of Sudan halt aerial bombardments immediately," the statement said. "We urge the Government of South Sudan to exercise restraint in responding to this provocation to prevent further escalation of hostilities."
It called for a resumption of negotiations by the Government of Sudan and the Sudan People's Liberation Movement-North.
President H.E. Saliva Kiir Mayardit has said he will not support armed opposition forces fighting against the government of Sudan, the South Sudan government website said.
Kiir said Sudan was threatening the sovereignty of South Sudan "through military invasion."
Liberation army members have clashed with the military of South Sudan, which separated from Sudan and became independent in July. Led by former officers of the southern army that fought neighboring Sudan in a 22-year civil war, the militias have taken up arms against their former comrades and become a challenge for the world's newest nation.