CAIRO — The first confirmed expansion of the coronavirus in Yemen was reported on Wednesday, prompting renewed calls from aid groups and the United Nations for a humanitarian cease-fire to help combat the infection at a time when the country is mired in increased fighting, widespread hunger and a major cholera outbreak.

The authorities in Yemen’s port city of Aden announced a cluster of five cases, up from the previous tally of one, and immediately imposed a two-week lockdown including the closure of shops, mosques and markets selling khat, a mild stimulant used by many Yemenis.

Although the cluster was tiny in comparison with neighboring countries, it confirmed the worst fears of health workers who have warned for weeks that even a mild coronavirus outbreak could quickly overwhelm Yemen’s devastated health system.