Heavy fighting between Sudan's army and former rebels in the south has forced the United Nations to evacuate staff.
It was the first sustained clashes between the two sides since a north-south peace deal last year.
The former rebel Sudan People's Liberation Army (SPLA) and the Islamist Khartoum-based government signed a peace deal in January 2005 ending Africa's longest civil, which killed two million people and drove four million from their homes.
"These hostilities constitute a serious violation of the security arrangements of the Comprehensive Peace Agreement," UN Secretary-General Kofi Annan said in a statement issued yesterday.
The United Nations has temporarily evacuated around 240 civilian staff from Malakal town close to the north-south border.
Mr Annan said UN commanders along with a delegation of SPLA and Sudanese army officers were now in Malakal in an attempt to calm the dispute.
The town was reported to be tense but quiet today.
