Foreign Office minister Kim Howells has criticised Israel's bombardment of Lebanon, while on a visit to Beirut.
He said Israel had not carried out "surgical strikes" and attacking the Lebanese nation was not the answer.
Downing Street said the prime minister would stand by Mr Howell's comments, adding the British government had "always urged restraint on Israel".
Meanwhile thousands of people across the UK have joined demonstrations against Israeli attacks on Lebanon.
Speaking in Beirut, Mr Howells said: "I very much hope that the Americans understand what's happening to Lebanon.
"The destruction of the infrastructure, the death of so many children and so many people. These have not been surgical strikes.
"And it's very difficult, I think, to understand the kind of military tactics that have been used.
"You know, if they're chasing Hezbollah, then go for Hezbollah. You don't go for the entire Lebanese nation."
Mr Howells said attempts were being made to engage with parties, but a demand for an immediate ceasefire would be a "meaningless gesture".
Responding to the minister's criticisms of Israel, Tony Blair's official spokesman said the prime minister would "not be unhappy about [Mr Howell's] comments at all".
He said Mr Blair, Foreign Secretary Margaret Beckett and government ministers were united in trying to find a solution to the crisis, and reiterated that "gestures" like calling for a ceasefire would not produce peace.
Southern Beirut has seen 10 days of air strikes
But Lt Colonel Yishou Efroni, of the Israeli Army, defended its tactics saying: "We told them to leave the towns and villages. We gave them notice of when we were going to attack.
"No army in the world is doing that. I didn't see the British or Americans doing that in Iraq warning the people to leave and then bombing."
He said people had been warned through the use of loud speakers and the dropping of leaflets from the air.
And Israeli government spokesman Avi Pazner told the BBC that Israel was not interested in invading, conquering or occupying Lebanon, from where it withdrew troops in 2000.
"We only want to get rid of Hezbollah," he said.
It is thought Israel wants to set up a deep buffer zone in southern Lebanon to try to stop Hezbollah from using the area to launch rocket attacks.
Earlier this week, Ms Beckett said while she had already condemned Hezbollah, her bowing to MPs' demands on criticising Israel was not the most effective policy.
Meanwhile former international development secretary Clare Short condemned the government's stance on the Middle East.
Protests have been held around the world
"We had a debate and most of those who spoke said that they thought Israel's response was disproportionate, that Britain should criticise, that there was a question of war crimes, that there should be a call for an immediate ceasefire," she told the BBC on Saturday.
Meanwhile in the UK, 11 rallies against Israel's actions were organised by groups such as Stop The War Coalition and the Muslim Association of Britain.
A rally to show solidarity with people in northern Israel - areas of which have been hit by Hezbollah rockets - is being held on Sunday by the Board of Deputies of British Jews.
Mr Howells spoke as he visited one of the last evacuation ships in Beirut, as the operation to get Britons out of the country wound down.
About 4,400 people have been taken out by the British; in total about 25,000 foreign nationals have been evacuated.
Mr Howells was there to inspect the final evacuation, and he praised everyone involved in the effort, saying it had gone smoothly.
Israel launched strikes against Lebanon following the capture of two Israeli soldiers by Hezbollah militants, who responded with rocket fire.
Shadow foreign secretary William Hague said the right of Israel to defend itself was "clear" and calls for an unconditional Israeli ceasefire were "futile" unless rocket attacks on it stopped and captured soldiers were returned.
But he said Israel's "disproportionate" response was delaying efforts to bring about a ceasefire.
Liberal Democrat foreign affairs spokesman Mike Moore MP said Mr Howells's suggestion that calling for an immediate ceasefire would be no more than a meaningless gesture was "extraordinary".
He said this had "drawn attention to the degree of isolation of Britain and United States from most of the rest of world opinion".