TEL AVIV, Israel (AP) -- More explosion struck Iran's capital Saturday after Israel said it was attacking the country.

The blasts happened after an earlier strike apparently targeted an area near the offices of Iran's 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 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."

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