GENEVA (Reuters) - The first vaccine campaign against cholera in Yemen has started, 18 months after war and a sanitation crisis triggered an epidemic, but the World Health Organization said it did not yet have permission nationwide to do the vaccinations.

FILE PHOTO: A woman sits with her sons while they are treated at a cholera treatment center in Sanaa, Yemen October 8, 2017. Picture taken October 8, 2017.REUTERS/Khaled Abdullah/File Photo

Some senior Houthi officials, whose forces control the capital Sanaa, have objected to vaccinations and this has already delayed the program by nearly a year, aid workers say.

There have been more than one million suspected cases of cholera in Yemen, and 2,275 recorded deaths since Nov 2016, the WHO says.

The oral vaccination campaign, which began in four districts in Aden on Sunday targeting 350,000 people, coincides with the rainy season, which health workers fear could spread the disease further.

“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,” Michael Ryan, WHO Assistant Director-General, told a news briefing on Monday.

“As of yet we don’t have established dates for those campaigns, but we are ready to move... just as soon as we get those necessary approvals,” he added.

WHO cholera expert Lorenzo Pizzoli said in a tweet from Aden on Sunday that the campaign hoped to cover at least four million people in areas at risk.

The disease is spread by faeces in sewage contaminating water or food, and it can kill because patients quickly lose fluids through vomiting and diarrhea.

Caught early it can be treated with oral rehydration salts.

“Cholera is still present and this is the classic situation where we’ve had a first big wave last year, and we fully expect another wave this year. That wave could be as large or potentially even larger,” Ryan said.

Water sanitation and treatment systems have been destroyed in many parts of Yemen, and access to health care remains extremely limited, he added.

Yemen’s war, a proxy conflict between Iran-aligned Houthis and the internationally recognized government of President Abd-Rabbu Mansour Hadi, which is backed by a Saudi-led alliance, has killed more than 10,000 people since 2015 and displaced more than two million.

In July 2017, the International Coordinating Group on Vaccine Provision - which manages a global stockpile - earmarked one million cholera vaccines for Yemen. But the WHO and local authorities together decided to scrap a vaccination plan on logistical and technical grounds and the doses were diverted to South Sudan.

The largest cholera vaccination drive in history, targeting more than two million people across Africa, is now being carried out in five countries - Zambia, Uganda, Malawi, South Sudan and Kenya - the WHO and GAVI vaccine alliance said on Monday.