The Israel Defence Forces (IDF) said its air force and artillery supported ground troops as they targeted Hezbollah locations in the border area of southern Lebanon which "pose an immediate threat to Israeli communities in northern Israel", including stockpiles and weapons.

Israel has declined to say how deep its troops will venture into Lebanon or how long the operation is expected to last.

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Sources told Sky News' US partner network NBC News the offensive will be limited in duration and scope, lasting for days, not weeks.

A spokesperson for the IDF said Israel's forces had "trained and prepared in recent months" for the ground operation, which was announced around 2am local time (midnight UK time).

Later, the Israeli military warned people to evacuate nearly two dozen Lebanese border communities.

Israel advised people to evacuate to the north of the Awali River, some 60km (36 miles) from the border and much further than the Litani River, which marks the northern edge of a UN-declared zone intended to serve as a buffer between Israel and Hezbollah after they went to war in 2006.

Hezbollah said it had launched rocket and artillery attacks against Israeli troops in Israel, but made no mention of Israeli forces operating inside Lebanon.

It marks the first time regular Israeli ground troops have entered Lebanese territory since 2006.

Israel had earlier launched small special forces operations into southern Lebanon ahead of a potential ground offensive, an Israeli official told NBC News.

It also began shelling border towns across southern Lebanon and sent an evacuation warning to residents living in the southern suburbs of Lebanon's capital Beirut, advising civilians living in specific buildings to leave ahead of airstrikes on the city.

The IDF also declared three small border communities to be "closed military zones", restricting access only to army personnel in the hours ahead of its incursion into Lebanon.

Shortly after the Israeli military announced troops had entered southern Lebanon, a military statement said approximately 10 projectiles had crossed into Israel from Lebanon. It said some were intercepted and others fell in open areas.

Shelling on the border areas meant UN peacekeepers could no longer carry out patrols "due to the intensity of the rockets going back and forth", it said.

The Lebanese Army - separate from Hezbollah - pulled troops back from checkpoints near the border amid intense shelling.

Thousands of people have fled Lebanon following the recent escalation of hostilities between Israel and Hezbollah.

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Battle 'may be long', Hezbollah deputy warns

Earlier, Hezbollah's deputy leader Naim Qassem said the group was ready for an Israeli ground offensive and the battle "may be long".

The UK's Foreign Secretary David Lammy reiterated calls on Monday for British nationals in Lebanon to leave the country, calling the situation in the country "volatile" and having the potential "to deteriorate quickly".

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The UK government has chartered a commercial flight out of Beirut to assist those who want to leave, scheduled to depart on Wednesday.

The Lebanese health ministry said more than 1,000 people have been killed across the country in the past two weeks due to Israeli airstrikes, and up to a million may now be displaced.