UNITED NATIONS — U.S. Ambassador Nikki Haley urged South Sudan's leaders on Tuesday to seize "the last chance" to salvage the 2015 peace agreement and end the worsening violence that has forced 4 million people to flee their homes and left 7.6 million in desperate need of aid.

Haley told the U.N. Security Council that "the people of South Sudan are suffering and the promise of their hard-fought independence is slipping away."

She said opposing parties in the world's newest nation must commit themselves to the revitalization process put forward by the eight-nation East African regional group known as IGAD "to resuscitate the peace agreement — and to do so quickly for time is running short."

There were high hopes that South Sudan would have peace and stability after its independence from neighboring Sudan in 2011. But the country plunged into ethnic violence in December 2013 when forces loyal to President Salva Kiir, a Dinka, started battling those loyal to Riek Machar, his former vice president who is a Nuer. The conflict has killed tens of thousands of people.

The August 2015 peace agreement has not stopped the fighting and clashes in July 2016 between supporters of Kiir and Machar set off further violence.

Haley's appeal to South Sudan's feuding leaders to support the High Level Revitalization Forum that IGAD is trying to convene to bring them together was echoed by nearly all council members and U.N. special envoy for South Sudan David Shearer.

Shearer painted a grim picture of the impoverished, war-torn nation as it approaches the end of the rainy season.

"The government appears emboldened by its recent military gains," he said. "Across the country, the opposition remains deeply fractured and has suffered significant military setbacks in recent months."

At the same time, Shearer said, South Sudan is "beset by social, economic and humanitarian challenges" and an economic crisis is fueling public frustration with many civil servants unpaid for over four months and salaries to security forces delayed. In addition, he said, deep distrust of military forces, "exacerbated by human rights abuses," is fueling the flight of many people to neighboring countries.

Shearer said the splintering of opposition forces has had "negative consequences" for the delivery of humanitarian aid. As an example, he said the World Food Program's regular convoys to Yambio — normally two days travel from the capital Juba — now require "13 separate permissions from armed groups along its route."

The United Nations has a 12,000-strong peacekeeping mission in South Sudan and the Security Council last year approved an additional 4,000 peacekeepers from the region to help protect civilians after a series of reported gang-rapes and other assaults when fighting erupted in Juba in July 2016.

Shearer told the council that deploying regional troops to Juba Airport, including at the mission's Tomping base, "remains a vexed issue." He said that he and the force commander are discussing "unresolved issues" with the government and will reach out to IGAD and the African Union to meet with the government in Juba as soon as possible.

South Sudan's First Vice President Tabo Deng Gai painted a much rosier picture in his speech to the General Assembly's annual ministerial meeting on Saturday, saying the government is confident that peace will soon return to the country "and wars shall be stories of the past."

This was in sharp contrast not only to the assessments of Shearer and Haley but to those of many council members.

In late April, Haley urged the council to impose an arms embargo and additional sanctions on South Sudan to pressure the parties to end the civil war — but Russia and China remain opposed.

Haley told the council Tuesday that the U.S. recently imposed its own sanctions on a number of South Sudanese officials "who thwarted peace."

"Our hope is that South Sudan's leaders will seize this opportunity" offered by IGAD, she said. "If not, we must resolve now both individually and collectively to do more to end this conflict."

Haley said President Donald Trump has asked her to visit central and east Africa in October and she will be in Juba soon.

Russia's deputy U.N. ambassador Petr Iliichev reiterated Moscow's opposition to targeted sanctions against individuals and an arms embargo, saying they "will not facilitate in alleviating the crisis. On the contrary, this will further compound the situation."

"Our hope is that preparations currently under way for the IGAD forum on revitalization of the peace agreement will lead to success," he said.