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

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.

Israeli Defence 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." 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 after Israel launched an attack targeting the country.

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

The US military declined to immediately comment on the attack.