Security tightened at US embassy

Large vehicles are being kept away from the US embassy in London

Security has been stepped up around the United States embassy in London.

Scotland Yard said the extra "high-visibility policing" was in response to "current concerns about US interests" rather than any specific threat.

The move emerged hours after Air France cancelled six flights between Paris and Los Angeles amid a security alert.

On Sunday, the US raised its alert status from Elevated to High Risk and warned terrorists were planning a September 11-style attack.

Last Friday, the Arabic satellite channel al-Jazeera broadcast a tape said to be of Osama Bin Laden's deputy, Ayman al-Zawahri, in which he vowed to defeat the US and warned that al-Qaeda was pursuing Americans "everywhere".

The additional security measures around the around the US embassy in London include restricting access for large vehicles to nearby streets.

Scotland Yard also called on "those people who visit, live and work in the capital" to be "vigilant and aware and report anything suspicious to police".

The fact that people are prepared to give their lives in terms of their so-called beliefs - that has raised the threat in an obvious manner Metropolitan Police chief Sir John Stevens

It would take "whatever action we believe is necessary to protect the public", the Yard said in a statement.

Earlier Metropolitan Police chief Sir John Stevens warned the public to remain vigilant to terrorist activity over the festive period, .

He said the use of suicide strategies meant the threat had taken a "quantum leap" since 11 September.

The UK has remained on its second highest level of alert for six weeks for "good reason", he told BBC Radio 4's Today programme.

But the threat did not compare to that being warned about in the US, he said.

Complacency warning

On Sunday, the director of homeland security in America, Tom Ridge, warned that terrorists were planning attacks on a scale similar to 11 September.

Sir John said that while he had not received warnings of that nature, the British public should not be complacent about the threat from terrorism.

"We've been used to Irish terrorism for somewhere in the region of 32 years, specifically in London," he said.

"But the threat is different in terms of suicide attacks... and the actual threat has taken a quantum leap since 11 September, everyone needs to know that."

Sir John Stevens said many terrorist plots had been foiled

Asked if he could quantify how the threat had increased, he said people had "just got to look around the world at places like New York, Istanbul, Bali".

"The fact that people are prepared to give their lives in terms of their so-called beliefs - that has raised the threat in an obvious manner," he said.

Sir John said there was an "immense amount" of activity underway to prevent attacks, and it would increase over Christmas and the New Year.

He insisted additional work by security services had "absolutely" prevented terrorist incidents, and praised the Met's relationship with intelligence services MI5 and MI6, which he said was "better than ever".

He said 90 people suspected of offences related to terrorism were currently going through the courts.