The strains burst into the open after US troops fired on a civilian vehicle, killing the driver, hours after seven Iraqi women and children were shot dead at a checkpoint. An Apache helicopter was also said to have blown up a lorry, killing 15 members of a single family, yesterday.

Such killings highlighted a series of military and political differences that senior British government sources say are creating “hairline cracks in the relationship”.

The military relationship has been strained by “friendly fire” deaths, an incident in which a Royal Marine commander complained that US troops endangered his men, and the Americans’ general attitude to the Iraqi population.

Politically, the allies have been at odds over the treatment of prisoners of war, plans for postwar Iraq and the Middle East peace process. Britain has also been dismayed by Donald Rumsfeld’s threatening noises towards Iran and Syria.

Monday’s checkpoint shootings were seen as a disaster for the coalition’s efforts to win Iraqi hearts and minds. Asked if they undermined attempts to court the local population, Colonel Chris Vernon, a British army spokesman, replied: “It does indeed, and if you were a civilian watching that you would interpret it in that way.”

The difference in approach was epitomised yesterday when the Royal Marines in four southern Iraqi towns swapped their helmets for berets as a sign of goodwill. American troops wear helmets at all times and checkpoint troops cover their faces with goggles and scarves.

US commanders are also said to have instructed their troops to adopt tougher tactics to weed out militiamen. “Everyone is now seen as a combatant until proven otherwise,” one Pentagon official is reported as saying before Monday’s checkpoint shooting.

British military sources spoke at length about the hard-won experience of UK troops from manning checkpoints and policing in Northern Ireland. “There is no doubt that with that experience, as well as in peace support operations in countries such as Bosnia, Kosovo and Sierra Leone, the British have learnt the art of restraint,” one source said.

“The Americans have got a more blanket approach to things,” said another. “You will never see their Marines wandering around in berets. They still wear hard helmets in Bosnia. You have got to be very careful you do not win the battle and lose the war. We have to be sensitive and we do not want to build up any resentment in the country.”

A senior American officer involved in war planning acknowledged yesterday that the US had misjudged the mood of the Iraqi people. “There is the information/psychological front that we try to push but we are probably not as sophisticated about it as we want to be,” he said. “There is a big cultural difference between the United States and the Arab world that makes it hard.

“Are we getting the message across to the educated people? We are. But to the people that want to be moved by the emotion and believe that there are no good motives and think that the United States are here for oil and only for oil, we have got to get the message across better.”

Tensions between the two countries’ forces had already surfaced after the deaths of three British servicemen in two “friendly fire” incidents after which one survivor accused an American A10 pilot of showing no regard for human life. A Royal Marine commander also accused the Americans of abandoning his men during a joint operation in southern Iraq on the first night of the war.

Further differences have emerged over the treatment of prisoners of war — though government sources said last night that Washington had now promised that all would be given the protection of the Geneva Convention.

But the Middle East is potentially the most divisive issue. Tony Blair has staked huge amounts of political capital to secure President Bush’s reluctant backing for implementing a new “road map” for the peace process to rebuild relations with Arab countries.

A key prime ministerial adviser said yesterday that if Mr Bush failed to fulfil his promises, that would represent a “significant breach which would change things in the future”. He added: “There are always stresses and strains in the structure of this relationship. There is no rift, but we are beginning to see hairline cracks.”

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