Britain is moving 700 troops to Cyprus to be ready for an emergency evacuation of UK citizens from Lebanon, as the prime minister urged those still in the country to leave immediately.
The government said military teams were moving there to further support British nationals in Lebanon, where Israel has been bombarding the south of the country.
The Royal Air Force also has aircraft and helicopters on standby to provide support if necessary.
It is the first phase of contingency plans for Lebanon, as the government seeks to avoid the chaos seen when British nationals were evacuated from Afghanistan in 2021.
The military teams will be supported by Border Force and Foreign Office officials.
Keir Starmer said the government was “ramping up the contingency plans” to rescue British nationals as he urged Israel and Lebanon to “pull back from the brink”.
Speaking from a flight to New York, where he will discuss Lebanon with world leaders at the United Nations general assembly, Starmer said: “The most important message from me this evening is to British nationals in Lebanon, to leave immediately and I just want to reinforce that.
“It is important that we be really, really clear: now is the time to leave. More broadly, I am worried about the situation and I think we need to be clear we need de-escalation, we need a ceasefire, we need to pull back from the brink.”
Lebanon’s health ministry has said at least 569 people have been killed and 1,645 injured, after a wave of Israeli airstrikes that left the country with its highest death toll since the end of the 1975-90 civil war.
It said the death toll includes 50 children and 94 women.
The new British troops add to a significant military presence of UK forces at RAF Akrotiri in Cyprus and the Royal Navy ships of RFA Mounts Bay and HMS Duncan.
The defence secretary, John Healey, left the Labour conference early and held a meeting with ministers, intelligence chiefs and diplomats on Tuesday afternoon to test government planning.
In New York, the US president, Joe Biden, addressed the UN general assembly on Tuesday and said a “full-scale war is not in anyone’s interest”.
“Even though the situation has escalated, a diplomatic solution is still possible,” he added.
The UN secretary general, António Guterres, told world leaders that Lebanon was on the brink of becoming a second Gaza, adding that the crisis had “become a non-stop nightmare that threatens to take the whole region down”.
The EU’s foreign policy chief, Josep Borrell, described the escalating conflict between Israel and Hezbollah as almost a “full-fledged war”.
They spoke as world leaders gathered in New York for the opening of the 79th UN general assembly, with diplomatic efforts having had little impact so far on the tensions and violence between Israel and Lebanon.