Intelligence agencies are investigating a series of internet warnings, said to be issued by al-Qaeda, about major terrorist attacks on the United States.
The messages - posted on several Islamist websites - include claims that an entire city could be destroyed.
This week, a statement, said to be from al-Qaeda intelligence services, warned the countdown to hit America had begun.
"It will be an even stronger strike than nuclear weapons so be prepared, oh mujahideen holy soldiers," it said.
The statement concluded that unless America and its allies withdrew from Iraq, Afghanistan and all Islamic countries, the organisation had ordered the elimination of US leaders and their supporters.
Another statement, repeated three times in recent days, was posted on an Islamist internet forum called the Mujahideen Network.
It claimed to be from the Islamic nation to the American people, and boasted that its group now had the ability to destroy an entire US city.
There are a lot of such messages on the internet - some not necessarily new - and their authors are hard to identify.
It makes it almost impossible to determine whether these really are final warnings before an attack, like those of 11 September, or just wishful thinking by al-Qaeda sympathisers.
But US and UK intelligence services are taking the messages, and their content, seriously.