"The size and scope of this challenge are immense," Ridge said. Comparing it to building the transcontinental railroad, fighting the Second World War or putting a man on the moon, he added: "The task before us is difficult, but not impossible."
Acknowledging the job would need unprecedented cooperation and communication among Government departments and agencies, Ridge cautioned, "The only turf we should be worried about protecting is the turf we stand on."
Bush and other key members of the Government have warned of possible reprisal attacks against Americans in response to the Administration's war on terrorism to punish the Islamic militants blamed for last month's attacks.
The number of possible targets are endless, from nuclear power plants to crowded subway trains.
Many Americans have been buying gas masks and antibiotics, fearing an attack with chemical or biological weapons.
"I know that many Americans at this time have fears," Bush said before he signed an executive order creating the homeland security post.
"We've learned that America is not immune from attack. We've seen that evil is real.
"America is going to be prepared."
But some experts and lawmakers say it is almost impossible to safeguard such a vast nation, and have urged Congress and the White House to give Ridge greater powers to mobilise a nationwide defence that would reassure the public.
"No homeland tsar can possibly hope to coordinate the almost hopeless dispersal of authority that currently characterises the 40 or more agencies ... with some piece of responsibility for protecting the homeland," warned former Senator Gary Hart, who co-chairs the US Commission on National Security for the 21st century.
Ridge will have a direct staff of 12, and about 100 more reporting to him from other federal agencies.
He will have an office in the West Wing, in the complex of offices belonging to White House Chief of Staff Andrew Card, and will report directly to Bush.
Despite heightened security after the attacks, especially at US airports, experts say the borders remain porous in some places, and it would be impossible to defend every potential target because there are so many.
Congressional investigators say federal, state and local health agencies are poorly coordinated and ill-equipped for another assault.
An attack using chemical or biological agents could strike thousands of Americans before being detected.
And experts say local doctors, nurses and emergency medical technicians have had little training with such diseases as smallpox and anthrax.
- REUTERS