The Saudi-led coalition in Yemen has authorised four U.S.-funded cranes to operate at Hodeidah port until Friday, United Nations aid agencies said on Tuesday, calling for the date to be extended to permit continued offloading of life-saving goods.

"We appeal to parties on ground in order to stave off famine that we can continue regularly to get food, medicines in, be it from humanitarian or the commercial side," Bettina Luescher of the U.N.'s World Food Programme (WFP) told a Geneva briefing.

A diphtheria outbreak in Yemen is "spreading quickly", with 678 cases and 48 associated deaths in four months, Fadela Chaib of the World Health Organisation (WHO) said, adding that a vaccine campaign had begun in the country.