A number of children have been killed in this week's fighting in South Sudan -- some in an attack on displaced civilians while others have been caught in the crossfire or died as a result of being recruited by armed groups and forces, the UN Children's Fund (UNICEF) said Friday.

The exact number of children killed is not yet known but they were among the dozens of civilians injured and many killed by gunmen on Thursday who attacked Internally Displaced Persons (IDPs) sheltering at the UN Protection of Civilians site in the central South Sudanese town of Bor, the UN agency said in a press release issued here.

"Utterly defenseless children were attacked in a place where they should have felt safe," said Jonathan Veitch, the UNICEF representative in South Sudan. "The trauma for children under such circumstances is indescribable."

During intense fighting earlier this week in the oil hub of Bentiu, when hundreds of children fled for protection onto a UN base, others were observed carrying weapons, uniformed in military units, as well as undergoing military training. UNICEF has credible reports that both sides to the conflict are recruiting children.

One 16 year-old boy who reported having been forcibly recruited three months ago by an armed group was gravely injured in the fighting, the UNICEF said, adding that his family brought him to the Protection of Civilians area at the UN base, where he was given medical treatment.

Meanwhile, a seven-year-old girl, who witnessed her family being killed in a hospital and a 14 year-old boy wounded in crossfire, are reported to have made it to safety at the UN Protection of Civilians area.

"This is fierce, brutal infantry fighting -- children must not be instruments of this conflict," said Veitch. "We talk of the long term damage done to children who are recruited, but right now the immediate threat is to their life."

"Those in positions of command and leadership have a duty to keep children out of harm's way and take all necessary measures to prevent children being part of armed groups and forces," he said.

Confirming the exact number of children recruited into the conflict in South Sudan is challenging, given the security situation in the country, though reports suggest very large numbers.

Under both international and South Sudanese national law, the forcible or voluntary recruitment or abduction of persons under the age of 18, whether as a member of a regular army or of an informal militia, is prohibited.

Since the fighting has started in South Sudan in December 2013, protection and human rights actors have documented a tragically significant increase in incidents involving children.

Girls and boys have been killed, maimed, raped, orphaned, recruited, and made homeless, the UNICEF said.

More than 1 million people have been displaced by the conflict in South Sudan.