The remarks, made in Beirut today by the Foreign Office Minister, Kim Howells, were the first public criticism of the US voiced by Britain. The Observer can also reveal that Tony Blair urged restraint in a private telephone convseration with the Israeli Prime Minister, Ehud Olmert, last week.

Sources close to the Prime Minister said that Olmert replied that Israel faced a dire security threat from the Hizbollah militia and was determined to do everything necessary to defeat it.

Britain's policy shift came as Israeli tanks and warplanes pounded targets across the border in south Lebanon today ahead of an immenently expected ground offensive to clear out nearby Hizbollah positions which have been firing dozens of rockets onto towns and cities inside Israel. Downing Street sources said Blair still believed Israel had every right to respond to the missile threat, and held the Shia militia responsible for provoking the cirisis by abducting two Israeli soldiers and shelling Israel.

But they said they had no quarrel with Howells's scathing denunciation of Israel's military tactics. Speaking to a BBC reporter before travelling on for talks in Israel, where he will also visit missile-hit areas of Haifa and meet his Israeli opposite-number, Howell said: 'The destruction of the infrastructure, the death of so many children and so many people. These have not been surgical strikes. If they are chasing Hizbollah, then go for Hizbollah. You don't go for the entire Lebanese nation.'

He added: 'I very much hope that the Americans understand what's happening to Lebanon.' Only hours earlier, President Bush used his weekly radio address to place the blame for the crisis squarely on Hizbollah and their Syrian and Iranian backers. He said that Secretary of State Condoleezza Rice, who is due to leave for the Middle East today, would 'make it clear that resolving the crisis demands confronting the terrorist group that launched the attacks and the nations that support it.'

Blair is scheduled to meet the President in Washington at the start of a US visit this Friday.

Senior diplomats said it was highly unlikely there would be a major diplomatic move to restrain Israel's planned south Lebanon incursion at least until then.

An advance force of tanks moved across the border yesterday, backed by a fierce barrage of airstrikes, including a half tonne bomb dropped on a Hizbollah outpost. Israeli forces focused much of their fire on the village of Ma roun al-Ras, on the crest of a hill less than a kilometre across the border. It was swathed in thick swirl of smoke.

Specially armour plated D-9 bulldozeers have also been brought in to level networks of foxholes and underground bunkers dug by Hizbollah.

Israel's army chief-of-staff Dan Halutz told reporters in Tel Aviv on Friday any military incursion would be limited in scope. 'We will fight terror wherever it is because if we do not fight it, it will fight us. If we don't reach it, it will reach us,' he said. 'We will also conduct limited ground operations as much as needed in order to harm the terror that harms us.'

Warnings to civilians

Israeli radio broadcast renewed warnings for civilians to flee the area by 7pm local time, but reports emerged of Lebanese casualties, including a seriously injured Lebanese woman who was taken to a hospital in the northern Israeli town of Safed.

An adviser to the Israeli Defence Minister Amir Peretz told The Observer: 'We are finally going to fight Hizbollah on the ground. The Israeli people are ready for this and the Sunni Muslim world also expect us to fight Shia fundamentalism and we are going to deliver.'

But he added: 'We have no intention of conquering and holding territory. We plan to clean a strip a mile from our border of Hizbollah bunkers and rocket-launching sites... We will go in and then we will go out.'

The Israeli air force dropped leaflets on southern Lebanon this week telling residents to leave their homes to avoid getting harmed in the fighting. Among the hundreds of thousands fleeing the fighting, there were few able-bodied men of military age.

Ali Suleiman, 50, from a village a few miles outside the coastal city of Tyre said his eldest son had joined Hizbollah.'When he dies, I will send another son and another and another. Tell Mr Blair, Muslims are not afraid - not of bombs or ships or hunger. We get our power from God.' Hizbollah has operated freely in the border region since Israel withdrew six years ago and are believed to have amassed an arsenal of some 12,000 rockets.

More than a week of airstrikes have done little to prevent Hizbollah from firing rockets at areas in northern Israel, including Haifa, the country's third-largest city. Today, more than 65 rockets fell - a dramatic increase from the previous 24 hours and at least 12 Israelis were injured. Britain's decision publicly to break ranks with the Americans over Israel's military tactics will cause deep concern in Jerusalem and a senior Israeli diplomat was at pains last night to play down any suggestion of a rift.

He said any feeling in London that Olmert's response to the Blair telephone call was a rebuff would be inaccurate. 'The tone was very positive. We agree on all the major aspects of this crisis and we are greatly appreciative of Britain's position.'

The Israeli Prime Minister, the source said, was merely reflecting an 'absolute determination to deal with the threat which we face from Hizbollah and to see that the UN resolutions requiring it to be disarmed are finally carried through.' Senior British sources also stressed their unwavering conviction that Hizbollah, and their Syrian and Iranian supporters, were responsible for igniting the crisis. They added that both the Syrian and Iranian ambassadors had been called into the Foreign Office last week to drive that message home.