President Donald Trump said on Saturday that the United States had begun "major combat operations" in Iran.
"Our objective is to defend the American people by eliminating imminent threats from the Iranian regime," Trump said in a video shared on social media.
Earlier, Israel Defence Minister Israel Katz announced the launch of a "preemptive strike" against Iran "to eliminate any threats it faces".
He also declared an immediate state of emergency across the country.
A Sky News Arabia correspondent reported that Israel closed its airspace after the strikes.
Iranian media on Saturday reported four explosions in central Tehran, with several more reported in Dezful and Kharg Island.
With hours, Iran launched retaliatory attacks on Israel.
In a statement, the Israel Defence Forces said it is "operating to intercept and strike threats".
It added that "sirens were sounded in several areas across the country following the identification of missiles launched from Iran toward the State of Israel".
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