The deadly measles disease has spread among children in Hajjah province in northwestern Yemen, medical aid group Doctors Without Borders (MSF) warned on Tuesday.
"So many children are presenting with measles at Abs hospital" in Hajjah province, MSF Director Christos Christou said on Twitter.
"There is a gap in routine vaccination among children," he added.
The MSF director arrived in Yemen a few days ago to have a first-hand evaluation of the humanitarian and health situation in Yemen.
"I saw children with severe malnutrition whose suffering could have been prevented if their mothers had the services needed for healthy pregnancies, if they had access to proper food, and timely health services," he said.
The Yemeni government and right groups accuse Houthi rebels of banning vaccines against the measles and other diseases in areas under their control.
There was no comment from the Houthis on the accusation, but the rebels say they suffer a severe shortage of medicine and vaccines in their areas.
Yemen has been mired by chaos and violence since September 2014 when Houthi rebels captured much of the country, including the capital, Sanaa. A military coalition led by Saudi Arabia entered the war in early 2015 to restore the government to power.
The eight-year conflict has created one of the worst humanitarian crises in the world, with millions risking starvation.
