Fighting between Sudanese government forces and armed opposition fighters has left dozens dead in the war-torn state of Southern Kordofan.

The opposition rebel group Sudan People's Liberation Movement-North said Wednesday it carried out an attack on government troops near the border with South Sudan, killing at least 30 Sudanese soldiers.

But a government official rejected reports that the rebels had taken control of parts of the area, telling the semi-official Sudanese Media Center that government troops fought off the attack, killing 30 members of the SPLA and capturing two others.

Fighting between Southern Kordofan's ethnic Nuba rebels, who are seen as supporters of South Sudan, and government forces began to escalate in June, about a month before South Sudan split from the north and declared independence.

The Nuba fighters sided with the south during Sudan's 21-year north-south civil war.

Khartoum has repeatedly claimed that the South Sudanese military is supporting the rebels in the oil-rich border state, but Juba denies the charges.

At a Wednesday meeting on the sidelines of the United Nations General Assembly session, U.S. President Barack Obama urged South Sudanese President Salva Kiir to take action on reports that his government was sparking unrest in Southern Kordofan.

Pro-southern elements are also fighting Sudan's army in Blue Nile state.

The U.N. has raised concerns about the humanitarian impact of the fighting in Blue Nile and Southern Kordofan, while the United States says the fighting is an obstacle to improving ties with Khartoum.

Sudanese President Omar al-Bashir is wanted by the International Criminal Court for alleged war crimes, crimes against humanity, and genocide in Sudan's Darfur region. His government has been fighting rebels there since 2003.