"The prolonged exposure to war, stress and uncertainty means that many children are in a state of ‘toxic stress’.

"This is having immediate and hugely detrimental effects on children, including increases in bedwetting, self-harm, suicide attempts and aggressive or withdrawn behaviour," Monday's report said.

In the report titled "Invisible Wounds", Save the Children and its Syrian partners interviewed more than 450 children, adolescents and adults across seven regions in Syria.

Fifty-one percent of the interviewees said adolescents were turning to drugs to cope with the stress, while 71% said that children increasingly suffered from bedwetting.

Eighty-four percent of adults and almost all children said that ongoing bombing and shelling was the primary cause of psychological stress in children’s daily lives.

Carolyn Miles, the president and CEO of Save the Children, said that in Syria, three million children had been born since the start of the war.

“The children we spoke with in Syria are terrified to play outside, afraid to go to school, and soiling themselves when they hear a loud noise," Miles said.

Save the Children's Syria Director Sonia Khush said two-thirds of the interviewed children lost either one of their parents or siblings, and some Syrian children interviewed said they wanted to die so they could go to heaven.

Khush added that children also said they wished to be "hit by snipers because if they got injured they would go to the hospital and leave the siege and they would eat whatever they wanted."

Syrian children in Turkey

Miles told Anadolu Agency that they believed half of the Syrian children in Turkey were out of school and Save the Children focused on getting children back to school.

"If they are out of school and do not have a big set of skills, these kids get really depressed and they actually have suicide attempts. We are making sure that we have some skill building for teenagers," she added.

Khush said the Turkish government "has been very generous" in accommodating and allowing more than 2 million Syrians inside Turkey. "It is such a positive thing, and a huge responsibility to take on."

She added that Save the Children was working with the Syrian refugees in Turkey, mainly in the southern border province of Hatay.

"We run several schools [in the region]. After the initial school shift is over, we try to help them catch up on years of schooling that they have lost."

Khush said that a big number of the Syrian families faced economic difficulties and they tried to support the whole family to make sure children do not work and go to school.

"We are about to start working in Istanbul with the same idea which is to find those children and get them to school," she added.

Syria has been locked in a vicious civil war since early 2011, when the Assad regime cracked down on pro-democracy protests with unexpected ferocity.

Since then, more than a quarter of a million people have been killed and more than 10 million displaced, according to UN figures.