Israel is bracing for a major attack in retaliation for a double assassination in Beirut and Tehran over the last week.
World leaders have been urging both sides to step back or risk triggering a full-scale regional war.
But Israel warned it would go on the attack if struck by Iran or its ally Hezbollah in Lebanon.
Yoav Gallant, Israel’s defence minister, said: “We are ready to move quickly to attack or to respond. We will exact a price, as we have been doing in recent days from the enemy. If it dares to attack us, it will pay a heavy price.”
He was backed by Benjamin Netanyahu, the Israeli prime minister, who told his security cabinet: “I reiterate and tell our enemies: We will respond and we will exact a heavy price for any act of aggression against us, from whatever quarter.”
The US said it was sending an aircraft carrier and warplanes to the region for “every possibility”. The FCDO said the UK had sent “additional consular officials, Border Force and UK military personnel to the region”, but did not provide further details.
Tensions have increased following the assassination on Wednesday of Ismail Haniyeh, a top Hamas leader in Tehran, one day after an Israeli strike in the Lebanese capital Beirut killed Fuad Shukr, a senior military commander from Lebanese terror group Hezbollah. Both groups are backed by Iran.