The Israeli military on Tuesday said it had begun a "limited, localized" ground operation against Hezbollah targets in southern Lebanon.
It said it was striking Hezbollah targets in areas close to the Israeli border, and that air force and artillery units were carrying out attacks to support the ground forces.
"A few hours ago, the IDF began limited, localized and targeted ground raids," it said. "These targets are located in villages close to the border and pose an immediate threat to Israeli communities in northern Israel."
The incursion came shortly after it was approved by Israeli political leaders and marked a new stage in Israel's war against the Iranian-backed militant group.
It came after Foreign Secretary David Lammy on Monday urged Britons to leave Lebanon before a ground incursion was launched by Israeli forces.
Speaking to Sky News, Mr Lammy said the situation in Lebanon is "fast-moving" and ministers can't guarantee help "if things escalate in a major way over the coming hours and days".
He told reporters: "I have been urging since coming to office in July for British nationals to leave Lebanon. And indeed the previous government from October 2023 was urging UK nationals not to travel to Lebanon.
"Notwithstanding that we sent a rapid response team. 700 troops are in Cyprus. We will do all we can to assist people to get out. We have secured places on commercial flights that are flying tomorrow so that UK nationals can get out."
His warning came after a US official told the BBC's US partner CBS on Monday that Israel had notified Washington of its intention to launch a limited ground incursion in Lebanon.
US State Department spokesman Matthew Miller said officials had been "engaged in conversations" with Israel about "limited operations" near the border with Lebanon.
But he repeated that Washington would continue to seek a diplomatic resolution to the conflict, warning that military action could lead to "miscalculation and unintended consequences".
Lebanese media reported heavy artillery fire in southern border towns on Monday evening, with Lebanese Armed Forces troops said to be "repositioning" near the border.
The Israel Defense Forces on Monday evening ordered residents to evacuate three areas in southern Beirut "immediately". There were later reports of Israeli strikes hitting the capital’s southern suburbs.
Earlier on Monday, Israeli defence minister Yoav Gallant urged troops to use "all the abilities we have" to return displaced residents north of the border.
Speaking to soldiers assembled near the Lebanese border, Mr Gallant said the "elimination of [Hezbollah leader] Hassan Nasrallah is a very important step, but it is not everything".
It comes days after the Israel Defence Forces (IDF) confirmed that Nasrallah had been killed in an airstrike, raising fears of a wider conflict in the Middle East.
In a video posted online, Mr Gallant was heard telling troops: “We will use all the abilities we have and if someone on the other side did not understand what all the abilities mean - it's all the abilities.
"And you are part of this effort, we trust you to be able to accomplish anything."
He added that "everything that needs to be done - will be done" and that "we will use all the forces from the air, sea and land".
Elsewhere, the first apparent Israeli airstrike in central Beirut in a year of conflict flattened an apartment building on Monday as Hamas said one of its commanders has been killed in a separate attack in Lebanon.
The Israeli Defence Force did not provide immediate confirmation on the strike which hit a residential neighbourhood in the centre of Lebanon’s capital.
The airstrike killed at least one person and wounded 16, an official with Lebanese Civil Defence source told the Associated Press.
He said the person killed was a member of the al-Jamaa al-Islamiya, or the Islamic Group, a Sunni political and militant group that is allied with Hezbollah.
The Popular Front for the Liberation of Palestine militant group said three of its members were killed in the strike on the city’s Kola district, including two military and security commanders.
Hamas said one of its commanders in Lebanon, Fatah Sharif, was killed in a separate strike on the Al-Buss refugee camp in the southern port city of Tyre.
His wife and daughter were also killed. Sharif was a commander in Lebanon and a member of Hamas’s command outside the Palestinian territories, the group said.
On Sunday, the Lebanese health ministry said at least 105 were killed across the country in airstrikes.
They said two attacks hit near the southern city of Sidon, about 28 miles south of Beirut, killing at least 32. Separately, Israeli attacks in the northern province of Baalbek Hermel killed a further 21 and injured at least 47.
As well as strikes in the capital, local media reported dozens of strikes against civilian buildings in the region of Bekaa and southern Lebanon.
Iran has vowed a “decisive response” for the killing of Nasrallah in the Iranian-backed militia.
Hezbollah confirmed senior commander Ali Karaki and Nabil Kaouk, deputy head of the militant group's Central Council, were also both killed in airstrikes at the weekend.
Mr Kaouk became the seventh senior member killed in a little over a week by Israeli strikes, with several of Hezbollah's founding members dead.
Israel also claims that at least 20 other militants were killed, including the head of Nasrallah's security detail.
The Lebanese health ministry also reported at least 14 medical workers were killed over two days in the south.
Israeli tanks were seen close to its border with Lebanon on Sunday, with senior Israeli military officials not ruling out a possible ground invasion of the country.
The last time Israel launched a ground offensive of Lebanon was in 2006, but intense cross-border fighting with Hezbollah ended in a stalemate after 34 days.
Western leaders have urged both sides for restraint, amid fears of a large loss of life if a ground invasion were to go ahead.
As well as its urgent calls for UK citizens to leave Lebanon, the Foreign Office has issued a travel alert for 19 countries amid fears that conflict in Lebanon could escalate into a wider regional war.
British tourists have been warned to remain vigilant in hotspots including Egypt, Tunisia and Morocco.
