More than 10,000 children have been killed in the Syrian civil war, the United Nations says, while many more are subjected to "unspeakable" suffering, including rape, torture and recruitment for combat.

Thousands more have also been forced to flee their homes during the three-year conflict, according to a report released this week to the Security Council and posted on the UN website.

Many more are the victims of the use of heavy weapons by government forces and allied militias in their fight with rebels who oppose the regime of president Bashar al-Assad, the report says.

However, it adds, both sides in the conflict are guilty of abusing and misusing children, with even the Free Syrian Army - the so-called moderate opposition - recruiting them for combat roles.

The report says methods of torture inflicted on children include beatings with metal cables, whips and metal batons; electric shock, including to the genitals; the ripping out of fingernails and toenails; sexual violence, including rape or threats of rape; mock executions; cigarette burns; sleep deprivation; solitary confinement; and exposure to the torture of relatives.

It cites a 16-year-old boy as saying he witnessed his 14-year-old male friend being sexually assaulted and then killed.

During the first two years of the conflict, most killings and maiming of children were attributed to government forces, the report says.

However, opposition groups had increasingly engaged in such acts in 2013 owing to increased "access to heavy weapons and the use of terror tactics".

On receiving the document, UN secretary-general Ban Ki-moon urged "all parties to the conflict to take, without delay, all measures to protect and uphold the rights of all children in Syria".

"The present report highlights that use of weaponry and military tactics that are disproportionate and indiscriminate by government forces and associated militias has resulted in countless killings and the maiming of children, and has obstructed children’s access to education and health services," he said.

"Government forces have also been responsible for the arrest, arbitrary detention, ill treatment and torture of children.

"Armed opposition groups have been responsible for the recruitment and use of children both in combat and support roles, as well as for conducting military operations, including using terror tactics, in civilian-populated areas, leading to civilian casualties, including children."

The report also notes that "all parties to the conflict have seriously hampered the delivery of humanitarian assistance in areas most affected by the fighting", and warns that children have experienced "a high level of distress as a result of witnessing the killing and injuring of members of their families and peers, or of being separated from their family and/or displaced".

Countries to increase intakes of Syrian refugees

The Federal Government last October announced it would resettle 500 Syrian refugees as part of its humanitarian refugee program.

The places were to be quarantined for Syrians within the existing annual refugee intake, which is capped at 13,750.

The United Nations says the Syrian crisis is a humanitarian tragedy which has killed more than 100,000 people and driven about 6.5 million others from their home.

Assad regime 'starving' Syrians US secretary of state John Kerry has accused Syria's Assad regime of blocking the delivery of food to communities.

More than 2 million refugees have fled Syria, many crossing the borders into Lebanon, Turkey and Jordan, and the international refugee agency UNHCR has called for the pressure on Syria's neighbours to be eased.

Immigration Minister Scott Morrison says the commitment to resettle the refugees was made in response to the UNHCR request.

He says the Government will work with the UNHCR to determine who gets the places, and that they will focus on highly vulnerable people in urgent need of protection.

US president Barack Obama's administration, meanwhile, has announced it has eased immigration rules to allow more of the millions of Syrians forced from their homes during the country's three-year civil war to come to the United States.

Only 31 Syrian refugees - out of an estimated 2.3 million - were admitted in the fiscal year that ended in October, prompting demands for change from rights advocates and many lawmakers.

And Britain announced in January it would give temporary residence to hundreds of vulnerable Syrian refugees, targeting those most traumatised by the war.

ABC/Reuters