AlQa'dah 04, 1436, August 19, 2015, SPA -- The United States has begun talks at the United Nations to sanction South Sudan if its government doesn't sign a peace deal with rebels within 15 days and all sides in the conflict don't promptly implement a cease-fire, President Barack Obama's national security adviser said Tuesday, AP reported. South Sudan President Salva Kiir has "yet again squandered the opportunity to bring peace," Susan Rice said in a White House statement. "The U.S. deplores this failure of leadership." In her statement, Rice said the U.S. has "initiated consultations" at the U.N. "to sanction those who undermine the peace process, if an agreement is not signed by the government within 15 days and a ceasefire is not implemented promptly by all parties." Kiir on Monday surprised the U.S. and others by refusing to sign a peace agreement with rebel forces despite a deadline set by the international community, saying he needs another 15 days. Mediators said rebel leader Riek Machar signed the accord. On Tuesday, a U.S. deputy ambassador to the U.N., David Pressman, called the failure to sign the peace deal "outrageous" in a speech to the Security Council. Both the U.S. and Britain said the Security Council must act to pressure those preventing an end to the conflict that has killed thousands and forced more than a million to flee their homes. If South Sudan's government will not sign the peace deal, "then we must all be firm on our next steps," Britain's U.N. deputy ambassador Peter Wilson told the council. "We cannot sit by while leaders fight and their people's suffering grows." A spokesman for the U.N. secretary-general said in a statement Tuesday that Ban Ki-moon "expresses his strong hope that President Kiir will sign the agreement by the end of the 15-day deadline" and urges an immediate end to the fighting.