"The members of the Security Council strongly condemned the series of attacks launched July 20 by armed opposition forces, and also the July 23 attacks by the South Sudan forces, as they fought for control of Nassir town," Xinhua quoted a UN statement as saying.
Expressing "deep regret" at the lack of progress towards peace and reconciliation in South Sudan, the Council members reiterated their full support for the mediation efforts of the Intergovernmental Authority on Development (IGAD).
The Security Council "urged all parties in South Sudan to immediately cease hostilities and to resume comprehensive and inclusive peace talks", said the statement.
The council underscored that attacks against civilians and UN peacekeepers may constitute a "war crime", and those responsible "must be brought to justice".
UN urged all parties in South Sudan to immediately cease hostilities
The Security Council also "welcomed the IGAD announcement that peace talks would resume July 30 and reminded the parties of their June 10 commitment to reach an agreement on the formation of a Transitional Government of National Unity within 60 days".
Under the command of former South Sudan vice president Riek Machar, the opposition forces attacked Nassir in the Upper Nile State July 20.
It was the first major attack since the South Sudanese government and opposition re-committed to a ceasefire agreement in May.
Last year, political infighting between President Salva Kiir and Vice-President Machar, who is accused of leading a military rebellion against the government, turned into a full-fledged conflict.
Kiir and Machar signed an agreement in Addis Ababa, Ethiopia, May 9, which stipulates comprehensive ceasefire and formation of a transitional government to arrange for general elections within a year from the signing of the deal.
However, clashes between the two sides were reported even after the signing the agreement.