UNICEF has said it is "extremely concerned" over the use by South Sudanese government troops of school supplies and backpacks that were supposed to be destined for children.

The complaint came after large numbers of heavily-armed South Sudanese government soldiers were photographed kitted out with bright-blue UNICEF schools bags on their shoulders.

Thousands of people have been killed in South Sudan and more than 800,000 have fled their homes since fighting erupted in mid-December.

The violence was triggered by a power struggle between President Salva Kiir and rebels led by former vice president Riek Machar.

UNICEF spokeswoman Sarah Crowe said they were "extremely concerned to see this flagrant abuse of UNICEF education materials by combatants".

"A large amount of UNICEF supplies - along with humanitarian supplies from other organisations as well as stores from schools and hospitals - have been looted in many locations during the conflict in South Sudan," she added.

"Such thefts display a complete disregard for the principle of protection of civilians and respect for humanitarian work," Ms Crowe said.

She urged the warring factions to "take appropriate action against the theft and use of supplies that are intended for the welfare of civilians - especially children".

Meanwhile, the United States has urged South Sudan's leaders to implement a ceasefire from 23 January between the government and rebels as an advance team of regional monitors arrived in the country.

A State Department official also pressed for the quick release by the government of the last four of a group of 11 detainees held on suspicion of trying to stage a coup.

Seven of the political figures were released on 29 January, partially meeting a rebel demand at peace talks.

The team of monitors arrived in South Sudan at the weekend to observe a shaky ceasefire agreed by the government of President Kiir and rebels led by Mr Machar, whom he sacked in July for trying to seize power.

"We strongly support their efforts and urge the government of South Sudan and rebel leader Machar to facilitate their important work," the official said of the monitors.

"The expeditious release and transfer of all of the detainees would reduce tension and build confidence in an inclusive reconciliation process," the official added.