South Sudan rebels on Sunday accused the army of violating a ceasefire, just hours after it came into effect, by bombarding their positions along the White Nile river.

"A military convoy -- two barges, seven gunboats -- has been moving from Bor to Panijar. Whenever they see our positions on the banks, they shell," said rebel spokesman Dickson Gatluak.

"The cessation of hostilities started at midnight on Saturday but the government has broken it," he added.