The US military reserves have been mobilised by President George W. Bush for the first time since the 1991 Gulf War, and a national emergency has been declared.
Mr Bush authorised the Defense Department to call up to one million reservists to active duty for up to two years.
The Pentagon said 35,500 reservists would be immediately called to duty for homeland defense and to provide support for port operations, medical needs, engineering and general civil support.
While officials left open the possibility of calling more reservists to augment US forces overseas, the Pentagon must consult with the White House if the number exceeds 50,000, said Mr Craig Duehring, a Pentagon official in charge of reserve affairs.
The first partial mobilization of the reserves and state-controlled National Guard since the 1991 Gulf War came as the US military, with the blessing of the US Congress, prepared for war.
The US Senate voted 98-0 to authorize Bush to use "all necessary and appropriate force against those nations, organizations or persons" who carried out the suicide attacks Tuesday that destroyed the World Trade Center in New York and heavily damaged the Pentagon.
"We've now arrived at a point where our country itself, as well as American interests around the globe, have been and can be seriously damaged," Defense Secretary Donald Rumsfeld said.
"It's going to require, as the president said, a war, a major, long-range, broadbased sustained effort to see that this does not damage our way of life," he said.
In a videotaped message sent to US forces worldwide a day after the attacks, Mr Rumsfeld told the troops "that more, much more, will be asked of you in the weeks and months ahead.
"This is especially true of those who are in the field. We face powerful and terrible enemies, enemies we intend to vanquish so that moments of horror, like yesterday, will be stopped," he said.
Few details of the military preparations have been disclosed with Mr Rumsfeld insisting on a high degree of secrecy about US military movements and operations.
US military forces have been on heightened alert since the attack, with US fighter aircraft patrolling US skies and warships off both coasts.
Mr Rumsfeld said combat air patrols over major US cities have been pulled back but fighter aircraft are still patrolling the Washington-New York air corridor.
Fighter aircraft at 26 US bases are on "strip alert," prepared to scramble on 15 minutes' notice, he said.
The Coast Guard is stepping up protection of the nation's ports, paying special attention to the security of arriving cruise ships, Rear Admiral Dennis Sirois said.
At US military installations, alert levels were lowered a notch from "Delta," the highest alert level, to "Charlie," a senior defense official said.
Two naval task forces are in the general area of the Middle East. A battle group led by the aircraft carrier USS Enterprise, which was leaving the Gulf for home when the terrorist struck, was directed to stay in the region, defense officials said.
The United States normally has close to 30,000 troops stationed in the Gulf, primarily aboard naval ships, but also at air bases in Saudi Arabia and Kuwait and at a major naval base in Bahrain.
AFP