The United States called Wednesday for an immediate deployment of African troops to safeguard a fragile peace deal reached last week by warring sides in Sudan.

The ceasefire agreement, signed last week in Addis Ababa, was the fruit of weeks of mounting international pressure and shuttle diplomacy.

But fighting broke out again on Sunday, just hours after the accord was signed by President Salva Kiir and rebel chief Riek Machar.

It was the second time in the five-month conflict that a truce failed to stick.

Linda Thomas-Greenfield, Assistant Secretary for African Affairs, said troops can help ensure that peace holds this time.

"The signing of the peace agreement is only the first step," she said during an online press conference.

"We have to work closely with the leaders of the region to make sure that we get IGAD (Intergovernmental Authority on Development in Eastern Africa) troops on the ground, who will be put in the position so that they can monitor the agreement and ensure that anyone who is involved in breaking that agreement will be held responsible," she said.

Washington is seeking a UN resolution "that will allow these troops to deploy as quickly as possible," Thomas-Greenfield said.

"This is one of our highest agenda items right now in dealing with South Sudan," the US diplomat said.

"This is what the people of South Sudan deserve. They fought for 30 years for their independence."

Thomas-Greenfield warned of possible dire consequences should the shaky peace deal fall apart.

"There is a famine that is looming if this fighting does not stop. More than 900,000 people have been forced from their homes," she said.

"We have to work to ensure that the agreement take root and that we start moving forward to provide a peaceful solution for the people of South Sudan."

The war in the world's youngest nation has claimed thousands -- possibly tens of thousands -- of lives, with more than 1.2 million people forced to flee their homes.

The United States, a key backer of South Sudan's push for independence from Khartoum, has poured hundreds of millions of dollars in aid to the country since it split from Sudan in 2011. It lobbied intensely for the peace deal.

US Secretary of State John Kerry at the beginning of May was in Juba, where he actively lobbied for the peace deal, meeting with President Kiir and speaking by telephone with Machar.