The United States accused Sudan's government on Thursday of carrying out air strikes on civilians in the restive border states of Southern Kordofan and Blue Nile and said the attacks were compounding a crisis in the two areas.
Humanitarian groups have accused Khartoum of carrying out a series of bombing raids on civilians, including one last month on a refugee camp that wounded one boy and left 14 people missing.
In the latest incident, Samaritan's Purse, a Christian relief organization, said a Bible school in the Nuba Mountains was bombed. Two buildings were destroyed but the organization said no one was hurt.
Sudan's government has repeatedly denied carrying out bombing attacks on civilians.
"The United States strongly condemns the bombing by the Sudanese Armed Forces of civilian populations in Southern Kordofan and Blue Nile States in Sudan," the White House said in a statement.
"Aerial attacks on civilian targets are unjustified and unacceptable. Such attacks are a violation of international law and compound the ongoing crisis in these areas," the statement said.
In a separate statement, US ambassador to the United Nations Susan Rice said she was "outraged" by what she said was a "heinous" bombing of the school.
South Sudan seceded from Sudan in July under a 2005 peace deal that ended decades of civil war, but the two countries have remained at loggerheads over issues including oil, debt and fighting along the poorly drawn border.
Southern Kordofan and Blue Nile both border South Sudan and are home to tens of thousands of fighters who fought Khartoum as part of the southern army during the civil war.
The United States has said it fears a large-scale famine in the two border states and has sought to boost pressure on Khartoum to allow greater humanitarian access.