The twin specters of a widening regional war and intensified suffering of civilians loomed over the Middle East on Saturday as the Iran-backed Houthi militia in Yemen threatened to respond to American airstrikes, and a senior U.N. official warned of a “horrific” humanitarian crisis in Gaza that he said was hurtling toward famine.

An American missile strike, launched from a warship in the Red Sea, hit a radar station outside the Yemeni capital, Sana, early on Saturday. The solitary strike came about 24 hours after U.S.-led strikes against nearly 30 sites in northern and western Yemen that were intended to deter Houthi attacks on commercial vessels in the Red Sea.

Houthi officials tried to brush off the latest assault, saying it would have little impact on their ability to attack vessels in the Red Sea. The Houthis, who are backed by Iran, say their goal is to punish Israel for blocking humanitarian aid into Gaza — though Yemeni analysts say the crisis also presents the Houthis with a welcome distraction from rising criticism at home.

The greater risk is likely borne by ordinary Yemenis, already struggling through one of the world’s worst humanitarian calamities — a dubious distinction that now also falls to Gaza.