Rifts grew between the United States and its closest allies on Wednesday as Britain and other European governments refused to back its claim that Iran is planning to mount attacks against US and allied assets in Syria and Iraq.

The US State department said it was withdrawing all non-emergency staff from diplomatic missions in Iraq following warnings from Donald Trump's government about a threat from Iranian-backed militia groups there.

But in a sign of growing differences, the Foreign Office said it had no plans to follow suit and the ministry of defence defended a British general who appeared to question the Trump administration's claims that an imminent threat has emerged from Iran and militias that it backs in recent days.