Lebanon's prime minister said on Tuesday his country was facing "one of the most dangerous phases of its history", urging the United Nations to provide aid for one million people displaced by Israeli air strikes.

"Around a million of our people have been displaced because of the devastating war that Israel is waging on Lebanon," the National News Agency reported Najib Mikati as saying. "We urgently call for more aid to reinforce our ongoing efforts to provide basic support to displaced civilians," he added in a meeting with UN representatives.

The United Nations on Tuesday launched an urgent appeal for US$426 million for the country.

The Israeli army said its troops were locked in fierce clashes in Lebanon after launching a ground offensive on Tuesday, escalating the conflict after a week of air strikes that killed hundreds.

The ground assault came as Israel targeted south Beirut, Damascus and Gaza, despite widespread international calls for restraint to avoid a regional conflagration.

US Defense Secretary Lloyd Austin late on Monday gave Washington's backing to Israel "dismantling attack infrastructure along the border" in Lebanon.

His statement came as the Israeli military ordered people in three districts of southern Beirut to evacuate.

Lebanese group Hezbollah previously said it was "ready if Israel decides to enter by land".

Meanwhile, air raid sirens were activated in central Israel on Tuesday, as explosions were heard in Tel Aviv, with the military saying projectiles had been fired from Lebanon.

"Sirens sounded in central Israel following projectile launches that crossed from Lebanon," the military said in a statement.

And as Israel announced its ground raids, Syria's official news agency Sana said the country's air defence systems had intercepted three rounds of strikes in the Damascus area.

State television said an anchor was killed "in the Israeli aggression" on Damascus, while Sana reported three civilians killed and nine others wounded.

There was no immediate comment from the Israeli military, which has carried out hundreds of strikes on Syria in recent years. (AFP)