GENEVA — The United Nations said on Tuesday that it was suspending plans for a cholera vaccination campaign in Yemen — reversing a decision made a month ago — because the disease’s rampant spread and the ravages of war there would make such an effort ineffective.

Jamie McGoldrick, the United Nations aid coordinator in Yemen, said plans for preventive vaccination were being “set aside.” He attributed the change to obstacles in delivering vaccines in the middle of a conflict that has crippled the country’s health system and hampered access to some areas threatened by the contagious disease.

Christian Lindmeier, a spokesman for the World Health Organization, told reporters the vaccine doses originally designated for shipment to Yemen would probably be sent to other countries threatened by cholera, where they could be used more effectively.

The surprise disclosure, made at a regular news briefing at the United Nations headquarters in Geneva, came as the number of Yemenis afflicted with cholera reached 313,000 and the death toll exceeded 1,700.