Hand-washing to combat the spread of coronavirus is the order of the day, but it's an unaffordable luxury for millions in war-ravaged Yemen where clean water is dangerously scarce.

Yemen's broken healthcare system has yet to register any cases of the disease, but if the pandemic does hit, the impact will be unimaginable in a country where five years of conflict has created what the United Nations calls the world's worst humanitarian crisis.

Five years after a Saudi-led military coalition intervened in Yemen to support the government against the Iran-backed Houthi, some 80 percent of the population is in need of aid.

Doctors Without Borders (MSF) said it was concerned that many Yemenis have no access to clean water or soap.

"We are extremely worried," Caroline Seguin, MSF's head of programmes in Yemen, Iraq and Jordan, told AFP.

"We can recommend they wash their hands, but what if they don't have anything to wash with?"

Nearly 18 million people, including 9.2 million children, do not have regular access to safe water, according to the UN's children's agency.

And only one third of Yemen's population has access to piped water, UNICEF said.

Eleven-year-old Mohammed's family, who live in the rebel-controlled Hajja province north of the capital Sanaa, are among those for whom water does not come out of a tap.

He and his sister leave their home on the back of a donkey every morning to retrieve supplies from a murky well three kilometres from their home.

"I get the donkey ready... and then head out at 7:30am, and I keep going back and forth until 10am," Mohammed told AFP.

The two children wait for their turn to fill up plastic canisters with a dirty hose.

Their family has no choice but to drink the contaminated water and use it for cooking.