The World Health Organization on Monday urged Yemen to approve cholera vaccinations it has offered to help contain an epidemic that could affect nearly a million people by year's end.

Yemen, where a multinational conflict has caused a humanitarian crisis, had asked the UN health agency earlier this year for doses of the vaccine, said Dominique Legros, the agency's cholera specialist.

The WHO sent a million doses in June only to see the Yemeni government change its mind, leading the United Nations to reassign the vaccines to Somalia and Sudan, Legros told reporters in Geneva.

Asked about Yemen's reversal, Legros said only that discussions with countries about vaccinations could be "complicated", noting the lack of familiarity with them in affected communities, especially in the case of newer vaccines like the one for cholera.

"We are still in negotiation with the government in Yemen to make sure we can also use (vaccines) to help control" the outbreak, he said.

Last week, the International Committee of the Red Cross (ICRC) said the rampant cholera crisis in Yemen had reached "colossal proportions", warning that it could affect 850,000 people by the end of the year.

More than 2,000 people have perished from the disease, according to the WHO.

The epidemic has put further strain on a ravaged health system in Yemen, where less than half of healthcare facilities are functioning as the conflict drags on.

Since March 2015, a Saudi-led coalition has been waging a war on behalf of the internationally recognised government against Iran-backed Huthi rebels.

More than 8,000 people have been killed, including at least 1,500 children, and millions displaced in the conflict which has pushed the impoverished country to the brink of famine.