Hezbollah said its fighters had fired a rocket targeting the Mossad spy agency headquarters near Tel Aviv on Wednesday and Israel unleashed more airstrikes on southern Lebanon in the heaviest bout of warfare between the arch-foes in a year.

The Israeli military said a single surface-to-surface missile was intercepted by air defence systems after it was detected crossing from Lebanon.

Warning sirens sounded in Tel Aviv, Israel's economic capital, but there were no reports of damage or casualties.

Israeli military spokesman Nadav Shoshani said he could not confirm what Hezbollah's target was when it fired the missile from a village in Lebanon.

"The result was a heavy missile, going towards Tel Aviv, towards civilian areas in Tel Aviv. The Mossad headquarters is not in that area," he said.

Warning sirens also sounded in other areas of central Israel, including the city of Netanya.

The Israeli military has been carrying out its heaviest airstrikes in a year of conflict this week, targeting leaders of the Iran-backed Hezbollah and hitting hundreds of sites deep inside Lebanon.

The hostilities started after war broke out last October between Israel and the Palestinian militant group Hamas in the Gaza Strip on Israel's southern border, with Hezbollah saying it was acting in solidarity with its ally Hamas.

Israel's focus has now turned to its northern frontier and southern Lebanon, already the scene of near-daily exchanges of fire for months.

There was no let up on Wednesday. Israel said its warplanes were currently carrying out extensive strikes in south Lebanon and the Bekaa Valley. Hezbollah has fired hundreds of missiles and rockets at Israel in recent days.

CASUALTY TOLL RISES

Hezbollah has blamed Mossad for the recent assassination of its leaders.

It has also accused the spy agency of carrying out an extraordinary operation last week in which the communications devices of its members were booby-trapped and exploded, killing 39 people and wounding nearly 3,000 in the worst security breach in its history.

Since Monday morning, the Israeli offensive has killed 569 people, including 50 children, and wounded 1,835 in Lebanon, Health Minister Firass Abiad told Al Jazeera Mubasher TV.

Half a million people are estimated to have been displaced in Lebanon, Foreign Minister Abdallah Bou Habib said. In Beirut, thousands of people who fled from southern Lebanon were sheltering in schools and other buildings.

Also on Wednesday, the Israeli military said a drone crossing into Israeli territory from Syria was intercepted by fighter jets south of the Sea of Galilee.

The Islamic Resistance armed groups in Iraq said in a statement they had attacked a target in the occupied Golan Heights via a drone.

Israel military leaders have said they are prepared for a range of options as it battles Hezbollah, a more sophisticated, disciplined and experienced enemy than Hamas which was created by Iran's elite Revolutionary Guards to counter an Israeli invasion of Lebanon in 1982.

Hezbollah has proven resilient over its decades-long hostilities with Israel, recovering from heavy blows and defying superior firepower. Iran's Supreme Leader Ayatollah Ali Khamenei said on Wednesday that killing important Hezbollah figures would not bring it to its knees.

Israeli troops have been training for months for a possible ground operation inside Lebanon aimed at securing its northern border and enabling thousands of Israeli residents who fled for their safety to return to their communities, one of Prime Minister Benjamin Netanyahu's government's top war priorities.

Israel's close ally the United States is sticking by it despite its concerns about mass civilian casualties. While Arab states have condemned Israel's military campaigns, they have not taken strong steps to force it to rein in the most powerful military in the Middle East.

On Tuesday, a strike in Beirut killed senior Hezbollah commander Ibrahim Qubaisi, who headed the group's missile and rocket force.

'LEBANON IS AT THE BRINK'

The new offensive against Hezbollah has stoked fears that conflict between Israel and Hamas in Gaza is widening across the Middle East.

The U.N. Security Council said it would meet on Wednesday to discuss the conflict.

"Lebanon is at the brink. The people of Lebanon – the people of Israel – and the people of the world - cannot afford Lebanon to become another Gaza," U.N. Secretary General Antonio Guterres said.

Israeli Defence Minister Yoav Gallant said the attacks had weakened Hezbollah and would continue.

Hezbollah "has suffered a sequence of blows to its command and control, its fighters, and the means to fight. These are all severe blows," he told Israeli troops.