The number of children who have fled war-torn Syria hit 1 million on Friday, while 2 million children have been displaced within their homeland's borders by the conflict, the United Nations said.

"This one millionth child refugee is not just another number. This is a real child ripped from home, maybe even from a family, facing horrors we can only begin to comprehend," said Anthony Lake, head of UNICEF, in a statement.

Children make up half of all refugees from more than two years of conflict in Syria, according to UN figures.

Most Syrian refugees have found a haven in Lebanon, Jordan, Turkey, Iraq and Egypt, but they are increasingly fleeing to North Africa and Europe.

The UN's most recent figures show that some 740,000 Syrian refugees are under the age of 11.

"What is at stake is nothing less than the survival and well-being of a generation of innocents," said Antonio Guterres, the UN high commissioner for refugees.

"The youth of Syria are losing their homes, their family members and their futures. Even after they have crossed a border to safety, they are traumatized, depressed and in need of a reason for hope," he added.

Inside Syria, meanwhile, more than 2 million children have been driven from their homes in the face of the conflict, which has morphed into a vicious, sectarian civil war.

The UN says that more than 100,000 people have been killed in the conflict, which started after a crackdown on protests against the government of President Bashar al-Assad in March 2011.

According to the UN, some 7,000 of the dead were youngsters.

The UN also warned that refugee children face the threats of child labor, early marriage and the potential for sexual exploitation and trafficking.

More than 3,500 children in Jordan, Lebanon and Iraq have crossed Syria's borders either unaccompanied or separated from their families, it added.

Aid agencies have scrambled to meet the needs of refugee children, for example by vaccinating hundreds of thousands against measles and providing schools in camps.

The UN's refugee agency has managed to register by name all 1 million children who have fled the country, and also helps obtain birth certificates for refugees born in exile to ensure they do not become stateless.