Hundreds of handheld pagers exploded across Lebanon at nearly the same time on Tuesday. Nine people were killed and more than 2,750 were wounded.
Reuters reported that many of the devices exploded in the middle of the afternoon in a suburb of Beirut.
The area is a stronghold for Hezbollah militants, who have been exchanging fire with Israel across the border for months. The group confirmed that two of its members were killed.
Iran's ambassador to Lebanon, Mojtaba Amani, suffered a superficial injury. Iranian Foreign Minister Abbas Araghchi blamed Israel for the attack, calling it a "terrorist act of the Zionist regime."
US officials said they played no part. State Department Spokesperson Matthew Miller said, "The US was not aware of this incident in advance."
The Associated Press reported that many members of Hezbollah carry pagers instead of cellphones over concerns that phones can be tracked by Israeli intelligence.
In a statement, Hezbollah vowed to inflict "fair punishment on Israel." Israeli military officials declined to comment.