Israel launched a daylight attack Saturday on Iran’s capital, with a cloud of smoke rising from the city’s downtown. The first apparent strike happened near the offices of Supreme Leader Ayatollah Ali Khamenei.

The United States is participating, though it was immediately clear the full extent of their involvement, according to a U.S. official and a person familiar with the operation, who spoke on condition of anonymity to detail sensitive military operations.

It wasn’t immediately clear whether the 86-year-old Khamenei had been in his offices at the time. He hasn’t been seen publicly in days as tensions with the United States have grown. 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.

Trump had wanted a deal to constrain Iran’s nuclear program, and he sees an opportunity while the country is struggling at home with growing dissent following nationwide protests. Iran had hoped to avert a war, but maintains it has the right to enrich uranium and does not want to discuss other issues, like its long-range missile program or support for armed groups like Hamas and Hezbollah.

It wasn’t immediately clear if Iran would immediately strike back, but it had warned that American military personnel and bases spread across the region would be targets for any retaliation.

Israeli Defense Minister Israel Katz described the attack as being done “to remove threats.” He did not immediately elaborate.

In Tehran, witnesses heard the first blast by Khamenei’s office. Iranian state television later reported on the explosion, without offering a cause.

Sirens sounded across Israel at the same time as it closed its airspace. 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.”

More explosions struck Iran’s capital after Israel said it was attacking the country. Authorities have offered no casualty information from the strikes.

Meanwhile, Iran shut down its airspace and mobile phone services were cut.

The warning to pilots came out as explosions rang out across Tehran, the Iranian capital.

The U.S. military declined to immediately comment on the attack.