"Military action in Iraq is imminent," the Foreign Office said in a statement on its official website.

"There is an increased threat from terrorism to the safety of British nationals in neighbouring countries.

"There is also a risk of the use of chemical and biological weapons by Iraq in the event of hostilities."

In a separate statement issuing a global alert to Britons, the Foreign Office said: "The risk of indiscriminate terrorist attacks in public places, including tourist sites, will be especially high during military action in Iraq.

"You should be vigilant, take sensible precautions, be aware of local sensibilities, monitor the media, and check our travel advice for the country you live in or plan to visit," it warned British citizens.

US President George W. Bush formally declared diplomacy dead ahead of his 0100 GMT Thursday deadline for Iraqi President Saddam Hussein to go into exile or face war.

The US military said Britain and the United States had conducted wide-ranging air strikes in southern and western Iraq Wednesday, bombing long-range artillery and a surface-to-surface missile system that threatened coalition forces poised to invade the country from Kuwait.

The warplanes struck at least seven locations over a 10-hour period.

British and US officials said the strikes were within the scope of normal enforcement of a no-fly zone over southern Iraq, and the US Central Command said they were launched in response to Iraqi anti-aircraft artillery fire.

The British warning of imminent miltary action came in travel advice to citizens to avoid travelling to Jordan.

The statement said: "We advise against travel to Jordan. If you are already in Jordan, we advise you to consider leaving while air routes remain open, unless your presence is essential."

"If Jordanian airspace is closed, or flights are disrupted, we advise British nationals to adopt a low profile, minimise travel and practice sensible security precautions."

"We have ordered the departure of dependants of the staff of our embassy in Jordan. We are now at core staffing."

Britons living in Bahrain, Israel and the Occupied Territories, Jordan, Kuwait, Oman, Qatar, Saudi Arabia and Syria were warned that in the event of a chemical or biological attack, they should follow the local authorities' and emergency services' instructions.

WAR.WIRE