The Hezbollah leader, Hassan Nasrallah, did not say how his group would respond to attacks on hand-held devices, which killed dozens when pagers and walkie-talkies exploded.

Israel warned residents in the north to stay inside or remain near bomb shelters Thursday after the Israeli Air Force struck Hezbollah military targets inside Lebanon.

The absence of a full-blown war in Lebanon has been touted as a core U.S. achievement, but now Washington’s grip on the crisis appears more tenuous than ever.