More than 4.8 million children have been born into war since Syria's brutal conflict erupted nine years ago, the UN children's agency has said, adding that thousands of others were killed or wounded in the conflict.

"The war in Syria marks yet another shameful milestone today," said Henrietta Fore, UNICEF executive director, as the conflict entered its tenth year.

"Millions of children are entering their second decade of life surrounded by war, violence, death and displacement."

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The 2011 uprising had been brewing for decades under the autocratic rule of President Bashar al-Assad, who inherited power from his father, Hafez al-Assad, in 2010 after nearly 30 years at the helm.

On March 15, 2011, and following demonstrations elsewhere in the region, tens of thousands of Syrians began rallying peacefully in cities across the country to demand the removal of al-Assad.

In response, the government resorted to the use of force and labelled anyone who opposed it a "terrorist".

Authorities spared nobody in the ensuing crackdown with opposition leaders, activists and professionals all being targeted.

Those who escaped execution were arrested and tortured. Many of them have gone missing.

Citing data from 2014 when official monitoring began, UNICEF said that "more than 9,000 children were killed or injured in the conflict".

During the same time "close to 5,000 children - some as young as seven - were recruited into the fighting (and) nearly 1,000 education and medical facilities came under attack".

The agency warned that the "true impact of this war on children is likely to be more profound".

Ted Chaiban, UNICEF regional director for the Middle East and North Africa, said an ongoing offensive by government forces on the last rebel stronghold in northwestern Syria was having "severe consequences on children".

Fighting in Idlib since December 1 has displaced more than 960,000 people, including over 575,000 children, according to UNICEF.

And as a result of the wider impact of the conflict "more than 2.8 million children are out of school inside Syria and in neighbouring countries," the agency said.

Two in five schools cannot be used because they are destroyed, damaged, sheltering displaced families or being used for military purposes, UNICEF said.

"Violence is still very much raging across the country, you have countless civilians that are trapped in a warzone in northwest Syria," Rachel Sider, advocacy adviser at the Norwegian Refugee Council, told Al Jazeera.

"We're quite appalled with the inaction of world powers that have allowed this violence to spiral out of control and allowed warring parties to kill, maim and displace millions of Syrians across the country," she added.

Addressing warring parties, Fore said: "Our message is clear: Stop hitting schools and hospitals".

"Stop killing and maiming children. Grant us the crossline and cross-border access we need to reach those in need. Far too many children have suffered for far too long."