WASHINGTON (CNN) -- A call by U.S. President George W. Bush for Congress to give him the power to use the military in law enforcement roles in the event of a bird flu pandemic has been criticized as akin to introducing martial law.

Bush said aggressive action would be needed to prevent a potentially disastrous U.S. outbreak of the disease that is sweeping through Asian poultry and which experts fear could mutate to pass between humans.

Such a deadly event would raise difficult questions, such as how a quarantine might be enforced, the president said.

"I'm concerned about what an avian flu outbreak could mean for the United States and the world," he told reporters during a Rose Garden news conference on Tuesday.

"One option is the use of a military that's able to plan and move," he said. "So that's why I put it on the table. I think it's an important debate for Congress to have."

The Posse Comitatus Act of 1878 bans the military from participating in police-type activity on U.S. soil.

But Dr. Irwin Redlener, associate dean of Columbia University's Mailman School of Public Health and director of its National Center for Disaster Preparedness, told The Associated Press the president's suggestion was dangerous.

Giving the military a law enforcement role would be an "extraordinarily Draconian measure" that would be unnecessary if the nation had built the capability for rapid vaccine production, ensured a large supply of anti-virals like Tamiflu and not allowed the degradation of the public health system.

"The translation of this is martial law in the United States," Redlener said.

And Gene Healy, a senior editor at the conservative Cato Institute, said Bush would risk undermining "a fundamental principle of American law" by tinkering with the act, which does not hinder the military's ability to respond to a crisis.

"What it does is set a high bar for the use of federal troops in a policing role," he wrote in a commentary on the group's Web site. "That reflects America's traditional distrust of using standing armies to enforce order at home, a distrust that's well-justified."

Healy said soldiers are not trained as police officers, and putting them in a civilian law enforcement role "can result in serious collateral damage to American life and liberty."

People who catch the worst strain of avian flu can die of viral pneumonia and acute respiratory distress, according to mayoclinic.com.

The disease has killed tens of millions of birds in Asia.

Last week, the U.N.'s health agency, the World Health Organization, sought to ease fears that the disease could kill as many as 150 million people worldwide.

"We're not going to know how lethal the next pandemic is going to be until the pandemic begins," WHO influenza spokesman Dick Thompson said, according to The Associated Press.

The consequences of an outbreak in the United States need to be addressed before catastrophe strikes, Bush said.

The president said he saw things differently than he did as governor of Texas. "I didn't want the president telling me how to be the commander in chief of the Texas Guard," he said.

"But Congress needs to take a look at circumstances that may need to vest the capacity of the president to move beyond that debate. And one such catastrophe or one such challenge could be an avian flu outbreak."

Should avian flu mutate and gain the ability to spread easily from human to human, world leaders and scientists would need rapid access to accurate information to be able to stem its spread, he said.

"We need to know, on a real-time basis, the facts, so the world's scientific community could analyze the facts," he said.

Bush said he had spoken to Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, about work towards a vaccine, but that means of prevention remained a distant hope.

"I take this issue very seriously," Bush said. "I'm not predicting an outbreak, but just suggesting to you we ought to be thinking about it, and we are."

Absent an effective vaccine, public health officials likely would try to stem the disease's spread by isolating people who had been exposed to it. Such a move could require the military, he said.

"I think the president ought to have all options on the table," Bush said, then corrected himself, "all assets on the table -- to be able to deal with something this significant."

Katrina lessons

Bush began discussing the possibility of changing the law banning the military from participating in police-type activity last month, in the aftermath of the government's sluggish response to civil unrest following Hurricane Katrina.

"I want there to be a robust discussion about the best way for the federal government, in certain extreme circumstances, to be able to rally assets for the good of the people," he told reporters September 26.

Last month, White House spokesman Scott McClellan said Bush "wants to make sure that we learn the lessons from Hurricane Katrina," including the use of the military in "a severe, catastrophic-type event."

"The Department of Defense would assume the responsibility for the situation, and come in with an overwhelming amount of resources and assets, to help stabilize the situation," McClellan said.

The World Health Organization has reported 116 cases of avian flu in humans, all of them in Asia. More than half of them have been fatal, it said.

On Thursday, the Senate added $4 billion to a Pentagon spending bill to head off the threat of an outbreak of avian flu among humans. The bulk of the money -- $3 billion -- would be used to stockpile Tamiflu, an antiviral drug that has proved effective against the H5N1 virus -- the strain blamed for six deaths in Indonesia last week.

U.S. health agencies have about 2 million doses of Tamiflu, enough to treat about 1 percent of the population. The money added by the Senate would build that stockpile to cover about 50 percent of the population.