After six years of conflict in Syria, aid agencies and humanitarian groups say enough is enough.

Today marks six years since the civil war began, a civil war which has claimed at least 320,000 lives, contributed heavily to the largest mass movement of refugees since the Second World War, fuelled the ascendancy of the extremist Islamic State group, and seen the indiscriminate bombing of civilians, including with chemical weapons illegal under international law.

Aid agency Unicef says that on this anniversary, the situation for Syrian children was at its worst in 2016, with at least 652 kids killed last year - a 20 percent increase from 2015.

Unicef spokesperson Lachlan Forsyth said the situation is horrific.

"Children are being used and recruited to fight directly on the frontline. Increasingly, they're taking part in combat roles. In very extreme cases they've been used as executioners, prison guards or suicide bombers."

Forsyth said parties on all sides are causing damage, and they need to step aside.

There were at least 338 attacks against hospitals and medical personnel last year and 255 children were killed in or near a school.

"There's a lot of discriminatory attacks going on. Schools are being attacked. Hospitals are being attacked. People are still being put in harm's way when they're trying to get to and from school, going to get water," said Forsyth.

Lebanon World Vision spokesperson Patricia Mouamar told Rachel Smalley this morning that their surveying shows airstrikes are still the biggest cause of stress for the nation's youngest.

"What those children are going through is more than any child [should] have to bear in a lifetime," she said. "It's hell for them."

LISTEN TO RACHEL SMALLEY'S FULL INTERVIEW WITH PATRICIA MOUAMAR ABOVE

Today, a United Nations Independent International Commission of Inquiry on Syria charged that the Syrian government, led by Bashsar al-Assad, committed a war crime when it deliberately bombed water sources in December which cut off water for 5.5 million people.

The attack was one of several war crimes committed by forces loyal to al-Assad, the report said.

It also catalogued atrocities by Islamic State and the group formerly known as the Nusra Front, the two combatants designated as terrorists by the UN Other rebel groups were also blamed for displacing communities during their offensives.