Des Browne, the defence secretary, ordered preparations for an urgent evacuation. An estimated 10,000 British citizens and up to 15,000 dual nationals are in Lebanon, which has been bombarded for four days in an escalating crisis triggered by the abduction of two Israeli soldiers.

Illustrious was being readied for the mission in Gibraltar and will be joined by HMS Bulwark, an amphibious assault warship.

A Ministry of Defence spokeswoman said no decision had been made about the tasks the ships would perform. However, a growing number of tourists were trapped in Beirut.

Israel has dispatched a hit squad of special forces to hunt down Sheikh Hassan Nasrallah, the leader of the Hezbollah group that abducted the soldiers.

Military leaders believe that killing Nasrallah, who escaped unhurt when his home and office in Beirut were bombed on Friday, could help to bring a swift end to their biggest attack on Lebanon in 24 years. “It’s all a matter of intelligence,” said a military source.

Further groups of special forces have crossed into south Lebanon to target the launchers of more than 350 Hezbollah rockets that have rained down on Israel, killing four people and injuring 60. Two barrages struck the resort town of Tiberias on the Sea of Galilee yesterday and sent tourists fleeing.

Israel stepped up airstrikes on Lebanon yesterday, destroying offices in Beirut belonging to Hezbollah and its Palestinian ally Hamas, which is holding a third Israeli soldier hostage in Gaza. Early today jets pounded targets in a south Beirut suburb where Hezbollah leaders are believed to live.

Helicopter gunships fired rockets into the northern Lebanese city of Tripoli and at least 35 civilians died in airstrikes in the south, including 20 in a convoy fleeing the border village of Marwaheen.

A senior Israeli intelligence official said troops from Iran had helped Hezbollah to fire an Iranian-made radar-guided missile that damaged an Israeli warship on Friday night with the loss of four soldiers on board. A second land-to-sea missile was said to have sunk an Egyptian merchant ship with 12 sailors on board.

The crisis prompted world leaders meeting at the G8 summit in St Petersburg to issue appeals for restraint. US President George W Bush urged Israel to avoid civilian casualties, while Vladimir Putin, the Russian president, questioned Israel’s motives and called for a “balanced response”.

Israel deployed its Patriot defence system to intercept long-range missiles capable of striking Tel Aviv.

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