ADDIS ABABA, Ethiopia (AP) — South Sudan's rebel leader says attacks by government troops on rebel-held areas could cause the failure of a peace agreement between the warring sides.

Riek Machar told reporters in Ethiopia Monday that he is concerned the government is not respecting the cease-fire.

Both sides agreed to a cease-fire that took effect Sunday, but there are reported violations in the volatile states of Upper Nile and Unity.

Machar accused government troops of attacking despite the cease-fire and urged President Salva Kiir's government to control its troops. For its part, South Sudan's military also accuses the rebels of attacking and breaking the cease-fire.

It is not possible to independently verify the alleged violations.

The U.S. has threatened U.N. sanctions against those who violate the pact.