Children queue for water at a school in Yemen's capital Sanaa sheltering them and their families after the conflict forced them to flee their areas from the Houthi-controlled northern province of Saada August 4, 2015. [Photo/Agencies]

"Children are bearing the brunt of a brutal armed conflict which escalated in March this year and shows no sign of a resolution," the UN agency said.

"This conflict is a particular tragedy for Yemeni children ... (they) are being killed by bombs or bullets and those that survive face the growing threat of disease and malnutrition," UNICEF Yemen representative Julien Harneis said.

The report Yemen: Childhood Under Threat said the number of children recruited or used in the conflict had more than doubled to 377 so far in 2015 from 156 in 2014.

All warring sides in Yemen are increasingly using teenage boys - who see fighting as a way to support their families financially - to swell their ranks, UNICEF said.

A quarter of Yemen's health facilities - around 900 - have closed since March, while shortages of medicines and medical supplies have disrupted those that remain open, according to the UN body, which said the health system was "crumbling".

More than 2.5 million children under the age of 15 are at risk of contracting measles, while nearly 2 million are likely to suffer from malnutrition this year, almost one million more than in 2014, UNICEF said.

"I would sell everything I have to ensure my children's wellbeing... what really disturbs me is how difficult it has become to get proper medical treatment," Umm Faisal, mother of an 18-month-old baby in Yemen, told UNICEF.