The United Nations Security Council on Friday called for South Sudan’s feuding political leaders to resolve a mounting crisis and threatened targeted sanctions against perpetrators of ethnic violence. A statement approved by all 15 Council members pressed President Salva Kiir and the rebel leader, Riek Machar, a former vice president, to put into effect a cease-fire and form a transitional government by Sunday. The statement did not name potential targets of sanctions but cited “those who take action that undermines the peace, stability, and security of South Sudan.” In December, Mr. Kiir accused Mr. Machar of trying to oust him in a coup, which unleashed months of ethnic attacks and failed cease-fires. Nationwide, the upheaval caused by the violence has put almost four million people at risk of going hungry and about 50,000 children at risk of dying of acute malnutrition.