The UK is closely monitoring Beirut’s international airport amid fears it may be forced to close by escalating fighting between Israel and Hezbollah, which would probably lead to an evacuation of British and other foreign nationals from Lebanon.

Defence sources said a halt to commercial flights out of the country would be a “big trigger” to launch what would most probably be an international evacuation by sea, though for now an emergency rescue is not thought imminent.

Keir Starmer, the UK prime minister, called on Britons to leave promptly via commercial flights, and would not be drawn on describing how a rescue might be conducted if air travel was suspended.

“I have a very important message for British nationals in Lebanon which is: the time to leave is now. The contingency plans are being ramped up but don’t wait for those, there are still commercial flights. It’s very important that they hear my message, which is to leave and to leave immediately,” Starmer said.

Later Starmer, who is in New York for the UN general assembly, was asked if it might be necessary to deploy British troops in Lebanon to help with a rescue. “I’m not going to get into the details of evacuation plans,” he said, but added that ministers had “put contingency measures in place” – and warned that fighting in Lebanon was escalating “almost hour on hour”.

On Tuesday, Britain said it was moving 700 troops to Cyprus, including members of the Royal Marines, to “focus on contingency planning” and help those leaving by commercial routes. They would be supported by members of Border Force and officials from the Foreign Office to help with advice and immigration processing.

It is estimated there are about 6,000 British nationals and dual nationals, as well as their dependants, remaining in Lebanon. About 10,000 more have left in the last week. The number of US citizens was estimated at 86,000 in 2022, while French citizens in the country are thought to number about 23,000.

A No 10 spokesperson said: “We have always said we stand by Israel’s right to self-defence but the PM’s message is that we are concerned by the situation and we need to see an immediate ceasefire and a focus on an immediate political settlement.”

Israeli airstrikes over the past three days have been concentrated in Hezbollah’s stronghold in south Lebanon, aimed at forcing the Iran-aligned militant group to halt attacks on northern Israel. More than 500 Lebanese have been killed in the bombing.

However, the two sides appear to be gradually escalating, with Israel bombing the mountains north of Beirut and Hezbollah targeting Tel Aviv on Wednesday.

Israeli experts have accused Hezbollah of hiding long-range missile launch sites in areas near the capital. Any bombing in that area would represent a significant escalation.

The Lebanese carrier Middle East Airlines is still operating flights from Beirut to cities in Europe and Middle East, though many other carriers have suspended their services. However, it is not immediately clear how many seats are available.

British sources said the most likely evacuation route would be via sea to Cyprus, though the situation is described as changeable. Navy vessels available are the auxiliary ship Mounts Bay, a military landing craft that can carry several hundred, and the warship HMS Duncan, though merchant shipping could also be deployed.

That scenario would mirror the rescue from Beirut that was organised during the last Lebanese war in July 2006. Warships evacuated 4,500 British nationals, starting five days after Israel announced an air and sea blockade of Lebanon in response to a cross-border raid by Hezbollah.

An airlift is an alternative possibility if it is safe to conduct one. British troops could be deployed, with the permission of the Lebanese government, to secure a port or airport evacuation area as happened previously in Afghanistan. But there is no prospect of troops rescuing Britons from their homes in the country.

John Healey, the defence secretary, chaired a meeting with ministers, intelligence heads and diplomats on Tuesday afternoon to discuss evacuation plans. Discussions continued on Wednesday but there were no immediate plans to hold a Cobra emergency meeting, a probable prelude to an evacuation.

Labour ministers are anxious to avoid the chaos of the evacuation of Kabul in Afghanistan in the summer of 2021, when thousands of people crowded at the airport in the hope of escape, while the Foreign Office in particular was criticised for being unprepared to handle the crisis.

Rami Mortada, Lebanon’s ambassador to the UK, said his country would be ready to help with evacuations. “The situation is unfolding by the hour and it looks like a very worrying situation for us Lebanese and clearly for the expat community in Lebanon. We would be ready to provide all the assistance needed,” he said.