Israel launched what it described as a “preemptive” strike against Iran on Saturday, as tensions between Tehran and Washington over Iran’s nuclear and missile programs escalated sharply.

Israeli Defense Minister Israel Katz said the attack was carried out “to remove threats,” but did not immediately provide further details about the target or scope of the operation.

An explosion was heard in Iran’s capital, Tehran, according to witnesses. It was not immediately clear what site was hit, and there was no immediate comment from Iranian authorities.

The strike comes amid weeks of heightened friction between Iran and the United States over Tehran’s nuclear activities and ballistic missile development. Washington has deployed additional fighter jets and warships to the region in what U.S. officials describe as a bid to pressure Iran into reaching a new agreement.

Shortly after the reported strike, air raid sirens sounded across Israel. The Israeli military said the alerts were issued proactively “to prepare the public for the possibility of missiles being launched toward the state of Israel.”

Israel’s Airports Authority announced the country’s airspace had been closed to civilian flights as a precaution.

Meanwhile, the U.S. Embassy in Qatar implemented a shelter-in-place order for all personnel and advised American citizens in the country to do the same until further notice.