An Israeli airstrike killed four people in Beirut early Tuesday, the Lebanese health ministry said, rocking an already shaky ceasefire between Israel and Hezbollah.

The operation marks the latest threat to a fragile four-month truce during which both Israel and Hezbollah have accused each other of violating terms, and came the day after United Nations officials demanded “answers and justice” over the discovery of the bodies of 15 aid workers found in a mass grave in Gaza.

In a joint…