Britons in Lebanon were told the leave immediately on the “next available flight” on Friday after Beirut was rocked by explosions.
The warning by the British embassy came as the Israeli military carried out a strike on Hezbollah’s central headquarters, sending clouds of orange and black smoke billowing in the sky.
At least two people were killed and dozens were wounded, Lebanon's health ministry said.
In an urgent warning, the British embassy in Lebanon said: “British nationals in Lebanon should leave now. You should take the next available flight.
“We're working to increase capacity and secure seats for British nationals to leave.”
Brits were urged to register their presence in the country for updates on any potential evacuation efforts, with British officials already sending 700 troops to nearby Cyprus should a large-scale evacuation effort be required.
Israeli media reported that Hezbollah’s leader Hassan Nasrallah was the intended target of Friday’s strikes in Beirut's southern suburbs, which was not immediately confirmed by the Israeli army.
A source close to Hezbollah told the Reuters news agency that Nasrallah was still alive after the series of huge airstrikes.
Israeli Prime Minister Benjamin Netanyahu abruptly cut short a visit to the United States, where he had been speaking at the UN General Assembly, after the strikes, his office said.
Hours earlier, Mr Netanyahu had addressed world leaders vowing that Israel's campaign against Hezbollah would continue - diminishing hopes for an internationally backed cease-fire on the border.
Iran, which backs Hezbollah as its proxy in Lebanon, called the strikes a dangerous game-changing escalation that will "bring its perpetrator an appropriate punishment."
Over 700 people have been killed in Lebanon this week as tensions in the region increase, according to Lebanon's health ministry.
Israel has dramatically escalated strikes, saying it is targeting Hezbollah's military capacities and senior Hezbollah commanders.
Top Israeli officials have threatened to repeat the destruction of Gaza in Lebanon if the Hezbollah fire continues, raising fears that Israel's actions in Gaza since Hamas’s October 7 terror attack would be repeated in Lebanon.
The International Organization for Migration estimated Thursday that more than 200,000 people have been displaced in Lebanon since Hezbollah began firing rockets into northern Israel in support of Hamas after it stormed into Israel, sparking the Israel-Hamas war.
Lebanon says a total of 1,540 people have been killed within its borders in that time.
The United States, the UK and other allies have jointly called for a 21-day cease-fire, which has so far been rejected by Israel.
Lebanon's foreign minister said the country welcomed the cease-fire efforts, and decried Israel's "systematic destruction of Lebanese border villages."
Israeli military vehicles were seen transporting tanks and armoured vehicles toward the country's northern border with Lebanon, and commanders have issued a call-up of reservists.
Mr Netanyahu says Israel is striking Hezbollah "with full force" and won't stop until its goals are achieved.
News of Friday’s blasts came as Mr Netanyahu was briefing reporters travelling with him in New York.
Israeli army spokesman Rear Adm. Daniel Hagari said the strikes targeted the main Hezbollah headquarters, located beneath residential buildings.
Six buildings in the Haret Hreik neighbourhood of Dahiyeh were reduced to rubble, according to Lebanon's national news agency.
The blast rattled windows and shook houses some 18 miles.
Officials at a nearby hospital said they received at least 10 wounded, three critically, including a Syrian child.
The Pentagon said the US had no advance warning of the heavy strikes in Beirut.
Mr Netanyahu's comments have dampened hopes for an international call for a 21-day truce between Israel and Hezbollah to allow time for a diplomatic solution.
Hezbollah has not responded to the proposal.
Iranian-backed Hezbollah, the strongest armed force in Lebanon, began firing rockets into Israel almost immediately after Hamas' October 7 attack, saying it was a show of support for the Palestinians.
Since then, it and the Israeli military have traded fire almost daily, forcing tens of thousands of people to flee their homes on both sides of the border.
