SANAA (Reuters) - Aid agencies have launched a polio vaccination campaign in Yemen to inoculate up to five million children under the age of five across the impoverished Arab state whose healthcare system has been crippled by more than three years of war.

The three-day campaign which began on Monday is organized by the U.N. children’s agency UNICEF, the World Health Organization (WHO) and the health ministry of the Houthi movement which controls most population centers in the country where millions face starvation, making them more vulnerable to diseases.

“The campaign comes at a very critical time,” said Meritxell Relano, UNICEF’s representative in Yemen.

“The lack of a fully functional healthcare system and widespread malnutrition have drastically increased the risk of preventable diseases.

“It’s very important that all the parties to the conflict allow unhindered access to enable us to reach all children, from north to south, east to west, with this vital vaccine.”

A Saudi-led coalition intervened in Yemen’s civil war in 2015 against the Iranian-aligned Houthi movement to restore the internationally recognized government, causing the world’s most pressing humanitarian crisis.

Save the Children said last week that an estimated 85,000 children under the age of five may have died from extreme hunger.

Yemen was declared polio-free in 2009, but experts say conflict-affected countries are particularly at risk of outbreaks because of disruption to their health systems.

Yemen also carried out a vaccination campaign in February, 2017 against polio, which spreads quickly among children and can cause irreversible paralysis within hours.

In the Houthi-held capital Sanaa on Monday, nurses delivered oral vaccines to children on dusty streets, some of whom cried and tried to squirm away.

They marked the fingers of vaccinated children with black felt pens, and used white chalk on doors of houses that had been covered in the immunization drive.

“Thank you to this immunization team, who in these difficult times, when the people need every riyal, have decided to pass by every house, saving transportation fees and effort for the people,” Sanaa resident Abdo Ahmed Ali said.

“That’s why we must thank them from our hearts.”

Yemen is home to the world’s worst cholera outbreak, with 10,000 suspected cases reported per week, the WHO said last month.

It said 16 percent of Yemen’s cholera cases were in the main port city of Hodeidah, the focus of the war, where only half the health facilities are operational.