About 100 employees have signed a petition demanding an end to "attacks and incitement against al-Jazeera".

Washington called the report outlandish after it appeared in the Daily Mirror.

The UK has warned the media not to publish a document said to record the threat - which some correspondents say may have been intended as a joke.

Editors have been told they could be prosecuted under the Official Secrets Act if they break the ban and publish the leaked document.

Work stopped in Qatar as well as at its al-Jazeera's foreign bureaux in several cities for a 15-minute symbolic protest about the issue.

The station - which plans to start an English-language version next year - has been repeatedly accused by Washington of bias and undermining its "war on terror".

It is often first to broadcast statements by al-Qaeda leaders, footage of anti-US attacks in Iraq and apparently heavy-handed action by US forces.

Staff repeated calls for an investigation into the bombing of their offices in Afghanistan and Baghdad by US forces - which led the death of one of their colleagues, reporter Tarek Ayoub.

The US says the attacks were a mistake, but al-Jazeera insists the location of its offices was well known.