The United States is concerned by reports that SPLM/A-In-Opposition forces are detaining UN Mission support staff in Upper Nile State.
The staff was last Thursday traveling by boat on the Nile River when abducted by the opposition.
The US calls for free humanitarian access to reach people in need.
US Department Spokesperson John Kirby says attacks against UN staff could amount to war crimes.
The Spokesperson warns such acts can lead to sanctions.
The US is generally concerned with reports of fighting in Unity State between the opposition and government.