The first-ever cholera vaccination campaign has been launched in Yemen as authorities battle to contain what has become the biggest outbreak of the disease in the history of the world.

The campaign covers just four "hot" districts which are at a particularly high risk of an outbreak and fast spread of the disease, said the World Health Organization.

But WHO and its partners, including Unicef and Gavi, the Vaccine Alliance, still do not have access to the whole country and just 350,000 people, including pregnant women, will be vaccinated in this current campaign.

Michael Ryan, WHO assistant director-general, said the organisation was "ready to move" as soon as approvals were in place.

“We have plans in place for extending that to all of the at-risk zones and we are still negotiating with health authorities in the north of the country, in Sanaa, in order to plan those campaigns,” he said.