Aid organisation Save the Children is warning that thousands of children are likely to have been separated from their families as a result of continuing violence in South Sudan and many may be surviving on their own in very remote areas.
The fighting broke out two weeks ago in the capital Juba and has spread to many parts of the country, killing at least 1000 people.
The organisation says thousands of children are likely to have been separated from their families and some may have seen their parents being killed.
South Sudan only became independent from Sudan in 2011, after decades of conflict, the BBC reports.
More than 121,000 people fled their homes when fighting started, with the result that many families were split up, Save the Children said.
The charity said while many people had sought refuge in United Nations compounds or host communities in safer areas, others, including children, were hiding in swampy areas with no shelter where they would be forced to drink stagnant water.
"We are very concerned that we cannot reach other parts of the country where the fighting has been escalating and where the needs of children are rising sharply," Save the Children country director Fiona McSheehy said.
What began as a power struggle between rebel leader Riek Machar and President Salva Kiir has taken on overtones of a tribal conflict. The Dinka, to which Mr Kiir belongs, are pitted against the Nuer, from which Mr Machar hails.
The government has offered a ceasefire, but the army says its forces are still battling rebels over oilfields in the north.
East African mediators have given both sides until Tuesday to agree an end to hostilities.