Restrictions on imports in conflict-hit Yemen are hampering aid efforts and threaten the lives of tens of thousands of children, the UN warned Friday.

Yemen already imported 90 percent of its food requirements before the conflict began and much of its fuel, essential to deliver aid and keep critical health facilities running at a time of acute power shortages, the UN children's agency said.

"Fuel may run out in less than a week in Yemen," said UNICEF spokesman Christophe Boulierac.

"If restrictions on the commercial imports of food and fuel continue, then it will kill more children than bullets and bombs in the coming months," he said.

"Humanitarian access to many areas is extremely difficult and we are frequently being blocked by parties to the conflict from going to places where people need us most."

The spokesman said 120,000 children risked suffering from severe acute malnutrition over the next three months if health and hygiene services do not function normally and children are not vaccinated against diseases such as diarrhoea, pneumonia and measles.

Boulierac said 2.5 million children under five are at immediate risk of diarrheal and other communicable diseases with sanitation systems collapsing.

"Diarrhea directly leads to malnutrition and can subsequently lead to the death of children," he said.

"As parties to the conflict continue to restrict commercial imports of fuel and food, prices have skyrocketed and people cannot afford to purchase essential supplies," he added.

Saudi Arabia has led six weeks of air strikes on Yemen in support of exiled President Abed Rabbou Mansour Hadi.