South Sudan's army on Tuesday called for international monitors to observe a ceasefire, as it rejected repeated rebel claims of heavy battles in the war-torn country.

Both sides have accused each other of breaking the August 29 ceasefire aimed at ending a brutal 20-month civil war, and the United States has said it is preparing possible new UN sanctions for two more individuals for their role in the continued fighting.

Ceasefire monitors from the regional East Africa bloc IGAD, which led efforts to negotiate the peace deal, are meant to be monitoring the ceasefire, the eighth to have been agreed since war began in December 2013.

But IGAD monitors for now lack the resources to monitor the rival forces, fractured into multiple militia units, with the rebels themselves divided.

"We have asked for monitors from the United States or the United Nations to fill the gap, to observe the ceasefire to stop these accusations," army spokesman Philip Aguer told AFP.

The request for international monitors had been made by the army chief Paul Malong, he added.

The world's youngest nation, South Sudan descended into bloodshed in December 2013 when President Salva Kiir accused rebel chief Riek 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.

Aguer dismissed rebel claims late on Monday that government forces had attacked rebel positions for a fifth day in the battleground northeastern state of Upper Nile as "lies", and said that the army "remains committed to the ceasefire".

But rebel spokesman William Gatjiath Deng claimed helicopter gunships from neighbouring Uganda -- which sent in troops to back government forces, but who under the peace deal, must withdraw by October 10 -- had pounded their positions.

It was not possible to independently verify the reports.

Source: AFP