Mulugeta Ayene, Associated Press
South Sudan's rebel leader Riek Machar, center, shakes hands with South Sudan's President Salva Kiir, center-right wearing a black hat, after lengthy peace negotiations in Addis Ababa, Ethiopia Monday, Aug. 17, 2015. South Sudan's President Salva Kiir refused to sign a peace agreement Monday with rebel forces, saying he needs 15 days before he will sign, although rebel leader Riek Machar had signed the accord before Kiir refused.

UNITED NATIONS — 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.

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."

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.

Oil-rich South Sudan has been at war since December 2013, and multiple attempts at a peace agreement have failed. The ethnic character of the violence has alarmed observers, with ethnic Dinka followers of Kiir pitted against the Nuer of former vice president Machar.

Last month, a Human Rights Watch report said South Sudanese troops had committed war crimes in rebel-held areas, with one survivor describing how soldiers made her watch while they gang-raped her daughter and burned her alive.

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.

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."

In July, after more than a year of warnings, the Security Council imposed its first sanctions on six generals for fueling the fighting.

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.