"We are seeing children in battle, at check-points and unfortunately among (those) killed and injured," Julien Harneis, UNICEF's representative in Yemen, told AFP during a stop in Geneva.
Staff of the United Nations children's agency and its partners had estimated that around 30 percent of fighters in the armed groups were minors, he said.
In Yemen's tribal culture, it is common for boys to take up arms at a young age -- something that is having dire consequences amid the spiralling conflict.
"You can say that up to a third of fighters in the armed groups in Yemen are children," Harneis said.
Saudi-led forces launched air strikes last month as Shiite Huthi rebels advanced on Yemen's main southern city of Aden after seizing the capital.
President Abedrabbo Mansour Hadi fled Aden for Saudi Arabia during the Huthi advance and the city has since seen heavy clashes between pro- and anti-government forces.
Even when they are not on the front lines in the conflict, children are particularly vulnerable, Harneis said.
UNICEF has confirmed that 77 children have been killed and 44 others injured since March 26, the Franco-British national said, adding though that the true toll was likely far higher.
"There are children dying in bombings in the north... and by very intense battles in Aden and Daleh. All of the parties to the conflict are to blame," he said.
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- Spike in malnutrition -
In addition to the violence, already high malnutrition levels in Yemen are expected to soar.
"We are going to see a spike in malnutrition in coming weeks. Unfortunately, that is something we are sure of," Harneis said.
"Difficulties in accessing water, rising prices for supplies, the difficulty to move around the country... All of this combined with cuts in state-run services (means) we will again see ... hikes in malnutrition," he warned.
Lacking access to food could be catastrophic in Yemen, where chronic malnutrition levels last year already stood at a staggering 48 percent -- among the highest in the world, Harneis said.
The conflict will also lead to a decline in the number of children attending school in a country where one million school-aged children were already not receiving an education, he said.
In light of the desperate situation, UNICEF is hoping to send medical supplies, drinking water and hygiene products to Sanaa "today", Harneis said.
If the UN agency manages to get the plane into the country, it would mark the first flight to Sanaa carrying aid supplies since the latest round of hostilities began.
Harneis said he was in Geneva to tell diplomats what he and his colleagues are seeing on the ground, "in the hope the states will use this information to reduce the impact of the conflict on children."