As the war in Yemen enters its third year more than 1,500 children have been killed, while a similar number have been recruited in the fighting.
In total more than 10,000 people have been killed in the fighting.
The latest figures released by UNICEF also reveal that 2,450 children have been injured in the war, while violence has created a food security emergency.
Speaking to ITV News earlier in March, Disasters Emergency Committee (DEC) Chief Executive Saleh Saeed, who is originally from Yemen, and has recently returned from a visit to the war-torn country said families are having to make the "unbearable" decision between buying medicine or food for their children.
"Families are having to choose whether they buy medicine for one child or buy food for their family. Just imagine that as a parent, how difficult would it be to make those choices."
He added: "It's unbearable and heartbreaking to see those families having to make those decisions."
In peacetime, Yemen imported around 90% of its food, but now after months of blockades and aerial bombardment, half the population do not know where their next meal is coming from.
Half of Yemen's population lives on less than £1.60 per day.
Close to half a million children in the Middle East's poorest country are suffering from severe acute malnutrition, UNICEF said, raising the risk of famine in the impoverished nation.
Citing United Nations-verified data, the Falling through the Cracks report also noted that the country's health system is on the verge of collapse, leaving 15 million with no access to health care.
“The war in Yemen continues to claim children’s lives and their future,” said Meritxell Relaño, UNICEF Representative in Yemen.
“Relentless fighting and destruction has scarred children for life. Families have been left destitute and are struggling to cope.”
As well as a lack of food and health care, the country's education system has collapsed, with two million children out of school.
UN secretary-general Antonio Guterres is to chair a pledging conference for Yemen on April 25 in Geneva.
During a recent visit to Yemen, Mr O’Brien said he met senior leaders of the government and Houthi rebels who control the capital Sanaa. All promised access for aid.
“Yet all parties to the conflict are arbitrarily denying sustained humanitarian access and politicise aid,” he said.
Mr O'Brien warned that if that behaviour did not change “they must be held accountable for the inevitable famine, unnecessary deaths and associated amplification in suffering that will follow”.