Rebels in South Sudan attacked government positions in two oil-producing states on Tuesday, the country's defense minister said, indicating that a truce signed only days ago is faltering, despite growing international calls for peace.

Rebel forces shelled the town of Bentiu, the Unity State capital, for two hours on Tuesday, Kuol Manyang Juuk told The Associated Press on Tuesday. He said mortar shells launched by rebels also hit the town of Nasir in Upper Nile state. Government forces fought back and repulsed the rebels, he added.

"If Riek Machar is committed to peace then he is not in control of his forces," he said, referring to the former South Sudanese vice president who is now the leader of rebel forces trying to dislodge President Salva Kiir. "This is the reality of the situation. In that case we will not remain with our arms folded. If they attack we will reserve the right to move out and follow them to where they came from."

It was not immediately possible to get a comment from the rebels.

South Sudan, the world's newest country has been wracked by sporadic violence since December 2013, when a section of government troops in the capital, Juba, appeared to splinter along tribal lines. The fighting pits rebel forces led by Machar, an ethnic Nuer, against government troops loyal to Kiir, who belongs to the dominant Dinka tribe. The political rivalry between the two is widely believed to have fueled the current conflict.

Aid groups say the country faces a deepening humanitarian crisis. About $600 million is needed to stock up on supplies before the end of April when the wet season starts. Hundreds of thousands of people have fled their homes.

The U.N.'s agency for children said in a statement Tuesday that 300 more children were released from an armed group in South Sudan following the release two weeks ago of 249 others. UNICEF and its partners hope up to 3,000 children will be gradually released.