Israel launched a daylight attack Saturday on Iran’s capital, with a cloud of smoke rising from the city’s downtown. It wasn’t immediately clear what the target was. But the attack comes as the United States has assembled a vast fleet of fighter jets and warships in the region to try to pressure Iran into a deal over its nuclear program.

Israeli Defense Minister Israel Katz described the attack as being done “to remove threats.” He did not immediately elaborate. In Tehran, witnesses heard the blast. Iranian state television later reported on the explosion, without offering a cause. Sirens sounded across Israel at the same time. The Israeli military said that it had issued a “proactive alert to prepare the public for the possibility of missiles being launched toward the state of Israel.”

According to the reports, the US officials said they expect this attack to be far more extensive than the American strikes last June against Iranian nuclear facilities.

The US military declined to immediately comment on the attack.