Washington - US Secretary of State John Kerry rang the president of South Sudan on Thursday to warn him to respect an oft-broken ceasefire.

The UN Security Council is to meet on Friday to discuss the latest fighting, having already threatened to impose sanctions on any parties imperilling a deal to end the nearly two-year war.

Under the agreement signed by President Salva Kiir and rebel leader Riek Machar, a ceasefire was due to enter into force last weekend, but fighting has continued in Upper Nile state.

"Secretary Kerry spoke with president Kiir today and expressed concern regarding recent ceasefire violations," a senior State Department official said.

"President Kiir confirmed to the secretary that he is committed to the implementation of the peace agreement and the ceasefire."

"The secretary underscored that the United States would work to support the implementation of the peace agreement and continue to provide humanitarian aid."

The world's youngest nation, South Sudan descended into bloodshed in December 2013 when Kiir accused Machar, then his deputy, of planning a coup.

The violence has left tens of thousands of people dead and the impoverished country split along ethnic lines.

Over two million people have fled their homes in a war marked by gang rapes and the use of child soldiers.

At least 200 000 civilians are sheltering in UN bases.

The United States has presented a draft UN resolution imposing an arms embargo and targeted sanctions on individuals who block provisions of the peace accord.