He said the US Computer Emergency Readiness team had issued a "situational awareness report to industry stakeholders".

However, he said it had been issued out of what he called "an abundance of caution".

The warning said the threat called for attacks to begin Friday and run through the month of December.

It is described as an "aspirational threat" and the nature of the warning is particularly vague, the BBC's Guto Harri in New York says.