South Sudan’s President Salva Kiir should immediately resign, a rebel leader said Wednesday at a news conference in Nairobi, Kenya. The terms in office of the president and the country’s legislature expired the same day at midnight under the nation’s transitional constitution adopted in 2011, Riek Machar said. Failure to open the country to elections as dictated by this constitution could spark a revolution, Machar said, according to the Associated Press.

South Sudan, the world’s newest state, has been gripped by violence since December 2013, when Kiir accused Machar, once his vice president, of plotting a coup against him. The country has since broken down along ethnic fault lines, with thousands of people killed in violence and millions displaced from their homes. The United Nations recently said it would send a fact-finding mission to assess the source of violence in the region and hold accountable those deemed responsible.

Machar placed blame for the recent surge in violence in South Sudan on Kiir, whose government has been accused of various human-rights violations by the U.N. The rebel leader, whose forces have also been accused of human-rights violations, said that if Kiir did not resign by midnight with a plan to hold elections, the country’s citizens would have the right to rise up against the government. In March, the nation’s legislature granted Kiir a three-year extension as president, although Machar rejected its right to amend the constitution, AP reported.

Last week, the U.N. Security Council blacklisted six generals of both government and rebel forces in South Sudan. The individuals, who include neither Machar nor Kiir, will not be allowed to travel internationally and have had their assets frozen.

Thursday marks the fourth anniversary of South Sudan’s independence after a decades-long conflict within Sudan. U.S. National Security Adviser Susan Rice said Wednesday the current situation in the country stands in stark contrast to the joy that broke out following the nation’s independence in 2011, Voice of America reported.

“The government and rebels are committing appalling crimes against innocent women, children, and the elderly. President Kiir and Riek Machar and their cronies are personally responsible for this new war and self-inflicted disaster. And only leaders on both sides can end this violence,” Rice said.

Government and rebel forces have agreed to repeated ceasefires and held months of talks in Ethiopia, but so far those efforts have failed to thwart the country’s violence.