WASHINGTON - The United States on Thursday slammed reported air attacks by Sudan's fighter jets on civilian targets inside South Sudan as "unacceptable," urging the two sides to exercise restraint to avoid escalation of tensions.

"Such incidents are unacceptable and threaten to escalate tensions between the two states. Additionally, continued aerial attacks on civilian targets by the Sudan Armed Forces are deplorable and constitute violations of international law for which there must be accountability," US State Department spokeswoman Victoria Nuland said in a statement.

South Sudan said Thursday that two Sudanese fighter jets Wednesday dropped bombs in Pariang county inside South Sudan, damaging oil and water wells while Sudanese ground forces were massing in a nearby area.

Nuland said the US demands that "the Government of Sudan end these aerial bombardments and immediately allow humanitarian access to civilians in Southern Kordofan and Blue Nile."

The US urges Sudan and South Sudan to exercise restraint as they are "drifting toward greater conflict and away from the promises of peace and collaboration to which each committed in the Comprehensive Peace Agreement," she added.

"An end to the violence in Southern Kordofan and Blue Nile is critical to establishing peace both within Sudan and between Sudan and South Sudan," Nuland said in the statement.

The US continues to stress to the government of South Sudan " the need to respect the sovereignty of Sudan and immediately end any military support for Sudan People's Liberation Movement-North rebels."

The statement warned that the actions of both parties "are inflaming conflict, fueling mistrust, and imperiling the efforts of the African Union High-level Implementation Panel to facilitate agreements between the parties on outstanding Comprehensive Peace Agreement issues."

The Sudanese army said last Sunday that armed clashes broke out between its forces and South Sudan forces on the border. Sudan and South Sudan signed a security agreement on February 10 to avoid armed conflicts between the two sides, but they have so far failed to demarcate their joint borders.