Iran fired a barrage of ballistic missiles at Israel in a counter-strike on Friday, after an unprecedented onslaught hammered the Islamic republic's top military leadership and targeted its nuclear facilities and bases.

Iran said it had activated its air-defence system on Friday night and explosions could be heard on Friday night across Tehran.

Air raids sirens and explosions rang out across Israel after Israeli Prime Minister Benjamin Netanyahu took to the airwaves to warn he expected "several waves of Iranian attacks" in response.

Smoke could later be seen billowing above the skyscrapers in downtown Tel Aviv, as Iran's Revolutionary Guard said it had attacked dozens of targets in Israel.

Israel's firefighting service said on Friday its teams were responding to several "major" incidents resulting from Iranian missile attacks.

They included efforts to rescue people trapped in a high-rise building. Rescuers said 34 people had been wounded, including a woman in critical condition.

Even as the latest strikes started, Iran's ambassador to the UN said on Friday that 78 people had been killed and 320 wounded in the first wave of strikes. (AFP)