Sudanese rebels called on the Khartoum government on Saturday to resume peace negotiations in Addis Ababa after it declared a unilateral four-month ceasefire in Blue Nile and South Kordofan.
The two states, which both have large ethnic minority populations, have been wracked by rebellion ever since neighbouring South Sudan broke away in 2011 without any resolution of their grievances.
Five years of on-off conflict between the army and the rebel Sudan People's Liberation Movement-North (SPLM-N) have seen neither side gain a decisive upper hand but successive rounds of peace talks in the Ethiopian capital have broken up without a deal.
"We are calling upon the regime to send a delegation to Addis Ababa immediately to sit with African Union mediators to activate the ceasefire offer," the SPLM-N said.
"A mechanism for monitoring and observing the ceasing of hostilities needs to be reached within a comprehensive peace process."
Both sides have suffered heavy casualties since a previous ceasefire expired early this year.
In March, a government delegation travelled to Addis Ababa for negotiations with the rebels and opposition parties but they broke up without any breakthrough.
Army spokesman Brigadier Ahmed Khalifa al-Shami said the new ceasefire was a "gesture of goodwill to give the armed groups a chance to join the peace process and to surrender their arms."
But it had been widely anticipated. The onset of the rainy season leaves roads in Blue Nile and South Kordofan impassable, preventing military operations of any size.
The rebels called on the government to release all political prisoners as well students detained during recent campus unrest as a show of good faith.