The UK has stepped up preparations for a possible evacuation of British citizens from Lebanon in the aftermath of last week’s assassinations in Beirut and Tehran – killings blamed on Israel that threaten to transform the war in Gaza into a region-wide conflict.

The Foreign, Commonwealth and Development Office and the Ministry of Defence said in a statement on Saturday that consular experts, border force officials and military personnel have been deployed to the region as part of planning for “a range of possible conflict scenarios, and with regional tensions rising”.

Landing ship RFA Cardigan Bay and HMS Duncan are already in the eastern Mediterranean, and the air force is putting transport helicopters on standby, the statement said.

The UK is not expecting an imminent need to evacuate citizens from Lebanon, the Observer understands, but the decision to put forces on standby comes after a meeting on Friday between defence secretary John Healey and his Israeli counterpart Yoav Gallant, during a visit to the region by Healey and the foreign secretary, David Lammy.

On Saturday the government also repeated a call for British nationals in Lebanon to leave while “commercial options are still available”, joining the US and several other countries. Dozens of companies have suspended flights to Lebanon and Israel in the past week.

“I’m not feeling anxiety, but feel confused about what I should do. What’s the best option, should I stay or should I leave and try to start over somewhere else?” said Lebanese-Brazilian Bruna Hassan al-Safawi, 23.

Safawi’s home is not far from Beirut airport: as the country is surrounded by Syrian and Israeli territory, it is effectively the one route in and out of the country for most people.

“If I’m still seeing planes landing, it’s OK, I’m safe. If I stop seeing planes, I’m gonna start packing my things,” she said.

In Haifa, Israel’s northernmost city, on Friday, bars and restaurants that would normally be packed on a summer evening were near empty. “I am trying not to think about it and doing things like normal, or you will go mad with worrying,” said resident Layla al-Nasser, 26.

Fears that the war in Gaza is on the brink of morphing into a conflict that could consume much of the Middle East grew over the weekend after comments from Iran on Saturday in which the Islamic Republic said it expected its ally in Lebanon, the powerful Shia militia Hezbollah, would begin striking deeper inside Israel, and no longer be confined to hitting military targets.

Israel and Hezbollah have traded tit-for-tat attacks since the Lebanese militia began firing on Israel the day after Hamas’s attack on Israel on 7 October, ostensibly to aid its Palestinian allies. In parallel with the fighting in Gaza, the conflict on the blue line separating the two countries has steadily escalated over the past 10 months, with tens of thousands of people on both sides displaced from their homes.

Tensions reached unprecedented levels this week in the wake of the back-to-back assassinations of Fuad Shukur, Hezbollah’s second-in-command, and Hamas’s political chief, Ismail Haniyeh.

Israel has claimed responsibility for Shukur’s killing, a missile strike on an apartment building in Beirut on Tuesday evening that also killed four other people and injured about 70. Israel said he was to blame for a rocket attack on the Israeli-controlled Golan Heights last weekend that killed 12 children and young people playing football. Hezbollah has denied it carried out the attack.

Just hours later, Shukur’s killing was overshadowed by the news that Haniyeh had been assassinated during a visit to Tehran for the inauguration of Iran’s new president. Conflicting reports have emerged about how the killing was carried out: on Saturday, Iran’s Revolutionary Guards released a statement saying that the Qatar-based official was killed by a short-range projectile with a warhead of about 7kg.

Both Hezbollah and Iran have vowed revenge for the killings. The Revolutionary Guards statement said that Tehran’s revenge for the attack would be “severe” and at an “appropriate time, place and manner”, blaming Israel for Haniyeh’s death. Israel has not commented on the Hamas leader’s death, but has carried out targeted assassinations on Iranian soil in the past.

In a speech at Shukur’s funeral on Thursday, Hezbollah’s secretary-general, Hassan Nasrallah, delinked the Lebanese front from Gaza for the first time, saying “the issue has gone beyond the support front”, and that the conflict with Israel had “entered a new phase”.

The US, Israel’s most important ally, said on Friday it would move additional warships and fighter jets to the region to “protect US personnel and defend Israel” as the Iran-aligned “axis of resistance” readied its response to Haniyeh’s killing.

The fighting in the Gaza Strip has also drawn in militant groups in Syria, Iraq and Yemen, which have fired drones and missiles at Israel and US assets in the region. Tehran directly attacked Israel for the first time in April, after a strike it blamed on Israel killed several senior Revolutionary Guards at the Iranian consulate in the Syrian capital, Damascus.

The barrage of 300 missiles and drones was carefully telegraphed in advance, allowing Israel’s allies to mount an efficient air defence response. Iran’s course of action this time is expected to be stronger, possibly taking the form of a joint attack.

US president Joe Biden, who has pushed hard for a ceasefire in Gaza in recent months, said the killing was “not helpful”, in comments late on Thursday.

Meanwhile, fighting continues to rage in Gaza, where health officials in the Hamas-ruled area said on Saturday the conflict had now killed almost 40,000 people. The UN said this week that nearly 40,000 cases of hepatitis A had now been confirmed in the besieged Palestinian enclave, and that health conditions “continue to deteriorate”.

An Israeli drone strike in the West Bank, the other Palestinian territory, killed five people on Saturday. Israel said it had targeted a group of militants in the town of Tulkarem who were planning an attack on Israelis.

The conflict broke out after Hamas’s 7 October attack in which approximately 1,200 Israelis were killed and another 250 taken hostage. An initial ceasefire in November broke down after a week, and protracted talks aimed at a truce and hostage release deal since have so far proved unsuccessful.