Attacks could be mounted from US Navy aircraft carriers stationed in the Arabian Sea. The USS Enterprise, which has F-14 Tomcats and F/A-18 Hornets, is in the Gulf with the Fifth Fleet.
A 14-ship US naval battle team, including the aircraft carrier USS Theodore Roosevelt and a Marine Corps assault force, yesterday left port in Norfolk, Virginia, bound for the Mediterranean. The British Royal Navy's Illustrious, which carries RAF Harrier GR7s and Fleet Air Arm Sea Harriers, is on its way to the Gulf for a joint exercise with the Oman armed forces.
A Royal Navy submarine is also in the region. It carries Tomahawk cruise missiles, as do several submarines and surface vessels of the US Fifth Fleet.
A dozen B52 bombers, which can launch cruise missiles, are understood to have been moved to the American base on Diego Garcia, in the Indian Ocean. All official military websites associated with the island base have been shut.
The US Air Force and the RAF have aircraft in the Gulf region patrolling the no-fly zones over Iraq. They could be ordered into action against Afghanistan.
The build-up follows strong words from the Bush Administration which told Afghanistan's Taleban regime that it was time for "actions and not negotiations".
US officials are working overtime to build an international coalition against terror - one that will legitimise military strikes and lessen the threat of reprisals against America and its supporters.
Pakistan could also provide a base for any special forces operations. British, Australian and New Zealand SAS troops are reported to be preparing for a role in any American operation to capture chief terror suspect Osama bin Laden, once his location has been pinpointed by allied satellite and ground intelligence networks.
Aside from the air bases in Pakistan, the British RAF has two large airfields at Masirah, off the northern Oman coast, and at Midway, in the western Dhofar region, which could be used for any air campaign.
India has offered use of its air bases on the border of Pakistan. Uzbekistan has also offered the US bases at Termez and Dzhara, on the border with northern Afghanistan.
Map: Opposing forces in the war against terror
Pictures: Day 1 | Day 2 | Brooklyn Bridge live webcam
Video
The fatal flights
Emergency telephone numbers:
United Airlines
: 0168 1800 932 8555
American Airlines
: 0168 1800 245 0999
NZ Ministry of Foreign Affairs and Trade
: 0800 872 111
US Embassy in Wellington (recorded info): 04 472 2068
Victims and survivors
How to donate to firefighters' fund