Intelligence agencies are investigating a series of internet warnings, said to be issued by al-Qaeda, about major attacks on the United States. Messages posted on several Islamist websites include claims that an entire city could be destroyed. One message warns of an attack stronger than that of nuclear weapons.
BBC Security Correspondent Frank Gardner looks at the significance of the messages.
What sort of messages are being posted on websites?
The vast majority of messages being posted on Arabic-language internet chartrooms consist of innocent banter, often between bored teenagers. But on certain 'jihadi' (ie extremist) sites there is a plethora of angry rhetoric against America and its allies.
Sometimes a contributor using an assumed name will log on for a short time and post a message saying something like "the countdown to a mighty blow against the non-believers has begun" or "prepare yourself, oh mujahideen, for victory against the Zionist oppressors".
What could "a stronger strike than nuclear weapons" mean?
It is possible that this could refer to an earthquake. One jihadi contributor, Abu Muhammad al-Ablaj, has been quoted as discussing the triggering of explosions along America's continental fault lines.
Do we know the messages are genuine?
We do not. Those posting such warnings tend to use only aliases, often based on historical figures whom they see as playing a heroic role in Islamic history.
It is also hard for Western governments to work out where the messages were sent from, but most are thought to originate in public internet cafes in the Middle East and Asia - the cyber equivalent of the phone box on the street corner.
Experts do say, however, that there are certain clues about some messages which identify them as being more likely to come from al-Qaeda. These include phraseology, historic references and so on.
How seriously are the US and UK authorities treating them?
In the current nervous climate (with Britain and many countries still at a heightened state of alert), Western governments are taking the warnings seriously.
But without knowing a specific target, they are limited in what actions they can take.
Over the Christmas/New Year holiday period, US intelligence agencies received information that made them believe there was a risk of terrorists setting off a radiological dirty bomb in a US city, so they dispatched experts to those cities with detection devices. They did not find any bombs.
How important is the internet for al-Qaeda communication?
The internet is invaluable. Before the attacks of 11 September 2001, al-Qaeda's senior leadership preferred to communicate by satellite phone. Now they are much more wary.
They still broadcast their propaganda messages through physical audio tapes but they also use audio messages sent over the internet. They also use it on a daily basis to communicate with each other and their supporters.
The internet is now so well established in Arab and Asian countries that it is relatively easy for terrorists to hide their usage of it among ordinary members of the public.