Health workers are rushing to vaccinate more than half a million children in Nepal as fears grow that last month's massive earthquake has made youngsters more susceptible to disease.

UNICEF has warned of a race against time to prevent a deadly outbreak of measles in the impoverished Himalayan nation following the April 25 quake that killed 7,652 people in Nepal.

The UN children's fund, the World Health Organization and the Nepalese government are targeting the urgent inoculation of 500,000 children in the areas worst-hit by the quake.

"Before the earthquake, one in ten children in Nepal was not vaccinated against measles, so we're going to vaccinate half a million children in the coming weeks," Kent Page, UNICEF Nepal's emergency spokesperson, told AFP.

At a mobile vaccination unit Wednesday in mountainous Kotdanda, near the capital Kathmandu, a steady stream of women queued up, each carrying a baby or young child. The children received a medical checkup by female Nepali health workers before receiving their vaccinations.

"Many of the children are living outdoors, they're not getting the food they need, their sort of physical well-being isn't so good, so they're more susceptible to often fatal diseases like measles," Page said.

"We're doing this measles vaccination campaign to prevent any outbreak or spread of measles," he added.

Health workers have started by immunising children under five in makeshift camps that have sprung up in three densely-populated districts in Kathmandu Valley -- Bhaktapur, Kathmandu and Lalitpur.

The vaccination drive will eventually include 12 districts most affected by the disaster.