World leaders called on South Sudan's President Salva Kiir to sign the IGAD Plus peace deal which was agreed to by rebel leader Riek Machar and Former Detainees leader Pagan Amum.

IGAD gave Kiir fifteen extra days to consult on the deal after he declined to sign it in Addis Ababa, Ethiopia on Monday.

United Nations Secretary General Ban Ki Moon expressed "his strong hope that President Kiir will sign the agreement by the end of the 15-day deadline," read a statement from his spokesperson.

The statement said Ban called on "all belligerents to immediately cease all hostilities, uphold international human rights and humanitarian law, and extend their full cooperation to UNMISS and to the Humanitarian Country Team’s life-saving activities in South Sudan."

United States national security advisor Susan Rice said her country is "deeply disappointed that the government of South Sudan under President Kiir yet again squandered the opportunity to bring peace to their people by refusing to sign the agreement. The US deplores this failure of leadership."

"The US calls upon the government of South Sudan and President Kiir to sign the agreement before IGAD's deadline without reservation or condition," she said.

The IGAD partners' forum, which includes the governments of Italy and Norway, as well as the European Union and other international groups, echoed this sentiment. "The representatives welcomed the signing of the agreement by the SPLM/SPLA-IO, Former Detainees and the Other Stakeholders, and called on the Government to sign the Agreement as well," the groups said in a statement.

German Foreign Minister Frank-Walter Steinmeier said: "it is very important now, that President Kiir and his government officially accept the outcome of the negotiations and sign the peace agreement."

"The fact that an agreement could be negotiated shows that both parties are basically willing to settle the conflict," he added.

Chinese ambassador Jiou Jieyi said "the best solution would be to reach an agreement," AFP agency reported.

The British deputy ambassador to the United Nations Peter Wilson also expressed displeasure at the government not signing the IGAD-Plus deal.