Further Israeli strikes have hit the Lebanese capital of Beirut, as the US warned it had intelligence of an impending ballistic strike on Israel by Iran. 

The Israeli military said it had targeted areas in Beirut's southern suburbs on Tuesday, local time, as part of a "precise strike". 

Two security sources said two attacks were carried out, with a high-rise building in the Jnah area hit.

Hezbollah also announced on Tuesday that it had launched a fresh round of missiles toward the Sde Dov airbase on the outskirts of Tel Aviv.

The US had indications of an imminent missile launch from Iran, a senior White House official said, maintaining that a direct military attack on Israel by Iran would carry severe consequences for Iran.

Air raid sirens have sounded in several places across central Israel, including in Tel Aviv.

However Israeli officials said they had not detected any aerial threats from Iran "for now".

"We do not yet detect an aerial threat launched from Iran at this time," Israel Defense Forces (IDF) spokesperson Rear Admiral Daniel Hagari said.

"We have dealt with this type of threat before, and we will deal with it now as well."

The US embassy in Israel has directed all US government employees and their families in Israel, the West Bank and Gaza to shelter in place until further notice.

Towns across south Lebanon told to leave

Thousands more people have been ordered to leave their homes in southern Lebanon by the Israeli military as it conducts what it called a "limited" operation across the Lebanese border.

The Israeli military ordered more than 20 villages south of the Litani River in southern Lebanon to evacuate on Tuesday, foreshadowing further strikes on homes Israel said were being used by Hezbollah fighters.

A statement from Hezbollah said no Israeli troops had entered Lebanon, directly contradicting earlier comments by the IDF that a limited number of soldiers had crossed the border. 

The Hezbollah spokesperson warned that the group's strikes on Tel Aviv hours earlier were "only the beginning".

The Lebanese militant group said on Tuesday it had used new Fadi 4 missiles to attack the Tel Aviv headquarters of Israel's Mossad intelligence agency and miliary bases.

Air raid sirens blasted across Israel and videos on social media showed smoke in the aftermath of the rocket barrage into Israel.

Israel's ambulance service said two people had been wounded by shrapnel from the missiles.

The Israeli military extended safety restrictions across much of the country later on Tuesday, limiting gatherings and telling people to remain near bomb shelters.

The UN's peacekeeping mission in southern Lebanon — established in 1978 following the first Israeli invasion of the country — said it had been notified in advance of the Israeli operations. 

"Despite this dangerous development, peacekeepers remain in position," a statement from the United Nations Interim Force in Lebanon (UNIFIL) said. 

"We urge all actors to step back from such escalatory acts, which will only lead to more violence and more bloodshed."

UN chief calls for immediate ceasefire in Lebanon

United Nations Secretary-General Antonio Guterres appealed for an immediate ceasefire in Lebanon and for the sovereignty and territorial integrity of the country to be respected, UN spokesperson Stephane Dujarric said.

"An all-out war must be avoided in Lebanon at all costs," Ms Dujarric said in a statement, adding that Mr Guterres had earlier spoken with Lebanon's caretaker Prime Minister Najib Mikati, telling him the UN was ready to help those in need.

"The Secretary-General will continue his contacts, and his representatives on the ground will also continue their efforts to de-escalate the situation," Ms Dujarric said.

The United Nations had earlier warned Israel against a "large-scale ground invasion" of Lebanon.

The violence had already forced up to a million people to flee their homes, according to Lebanese officials.

More than 1,700 people have been killed in Lebanon since hostilities began last October 8 when Hezbollah began firing missiles into Israel the day after the Hamas attack on Israel, with most of the fatalities taking place since Israel intensified its campaign in mid-September.

"Not just Hezbollah, all of Lebanon will fight this time. All of Lebanon is determined to fight Israel for the massacres it committed in Gaza and Lebanon," said Abu Alaa, a resident of the southern port city of Sidon.

Turkey said it was ready to carry out evacuations of its nationals from Lebanon via air and sea.

In a statement, its foreign ministry said it was working with 20 countries on evacuation preparations that would see other foreign nationals evacuated via Turkey.

Israeli strikes on Syria kill six

A Syria war monitor said six people including three civilians were killed in Israeli strikes early Tuesday targeting Damascus and its vicinity, amid fears of broader Israeli raids on the country.

The Syrian Observatory for Human Rights said the strikes targeted two vehicles in Damascus's upscale Mazzeh district, as well as "air defence batteries at the [city's] airport, and the vicinity of the Kisweh area", south of the capital.

"Three civilians including a media worker" were killed, as well as three pro-Iran fighters, said the Britain-based Observatory, which relies on a network of sources inside Syria.

Syrian state media had earlier reported that three civilians were killed in Israeli air strikes and nine others wounded, citing a military source.

"The Israeli enemy launched an air aggression with warplanes and drones from the direction of the occupied Syrian Golan, targeting several points in Damascus," the official news agency SANA said.

"Three civilians were killed and nine others injured," it added.

Since Syria's civil war erupted in 2011, Israel has carried out hundreds of strikes in the country, mainly targeting army positions and Iran-backed fighters, including Hezbollah.

Israeli authorities rarely comment on individual strikes in Syria, but have repeatedly said they will not allow arch-enemy Iran to expand its presence there.

The strikes have increased in recent days, including on areas near the border with Lebanon.

Tens of thousands of people have crossed into Syria over the past week, fleeing heavy Israeli air strikes on Lebanon.

Reuters/AFP