The Pentagon is planning to take a larger role in responding to "catastrophic" events within the United States such as natural disasters and terrorist attacks and is developing plans to use active-duty troops to respond to an avian flu pandemic, the Defense Department's top domestic security official said Wednesday.

The lessons from Hurricane Katrina require that the military assume a greater role during major disasters, said Assistant Secretary of Defense for Homeland Defense Paul McHale. But McHale stressed that active-duty troops would be used only for "catastrophic events" and would not be called to respond to the more than 50 storms, floods and hurricanes that require federal disaster assistance each year.

"A catastrophic event is the kind of event where the destruction is so severe that we anticipate such an event will occur once or twice in a generation," McHale said. "A Category 4 hurricane of the type experienced with Katrina is a pretty rare event." President Bush said in a nationally televised address from New Orleans four weeks ago that he would seek a larger role for the active-duty military in responding to domestic disasters. His call followed criticism of federal hurricane response as slow and inept.

McHale's remarks, during a breakfast meeting with defense writers Wednesday, provided the first glimpse into the extent of the military's new mission. His comments also reflected military wariness about adding duties.

Some worry that active-duty troops - already stretched by protracted deployments in Iraq and Afghanistan - might be too readily assigned to laborintensive domestic assignments that traditionally have been the domain of state and local authorities. McHale emphasized that active-duty troops would not become "first responders" to every domestic disaster and probably would be needed only when local and state authorities were themselves disaster victims, as with Katrina.

Government officials have not decided what scope of disaster would trigger such military response. Still, McHale said that an outbreak of avian flu could be so severe that active-duty forces might need to help the National Guard enforce quarantines.

"It is conceivable that a ... biological event would be so large, so catastrophic, that every agency of the federal government, most especially to include [the Defense Department], would be involved in a comprehensive federal response," McHale said.

He said the Pentagon was drawing up plans for dealing with an avian flu outbreak and would soon detail its role in the overall federal response. Bush said last week that he would consider using troops to "effect a quarantine" in the event of an avian flu outbreak, but he gave few details.

With an annual budget of more than $400 billion and fleets of ships, helicopters and trucks at its disposal, the Pentagon is considered by many to be the only agency equipped to respond immediately to major national disasters.

The current National Response Plan for such disasters authorizes the federal government to send active-duty troops at the request of civilian agencies such as the Department of Homeland Security and its Federal Emergency Management Agency.

The U.S. Northern Command in Colorado, responsible for deploying troops domestically, is considering establishing a permanent rapid-reaction force in case of U.S. disasters. Some experts in and outside the Pentagon caution against overreacting to Katrina with drastic measures or changes in law.

The Posse Comitatus Act prohibits active-duty federal troops from enforcing the law domestically, though governors can summon National Guard troops for that purpose, and the president can temporarily override the prohibition by invoking the Insurrection Act. Some experts fear that making it easier for the president to send troops to maintain order could undercut protections against the use of the military within U.S. borders.

"There's a danger of swinging the pendulum too far to the other side because of what happened with Katrina," said Christine E. Wormuth, a senior fellow at the Washington-based Center for Strategic and International Studies. "Those guiding principles are there for a reason. We need to be careful about making changes to them.

" The Insurrection Act was used most recently in 1992, when California Gov. Pete Wilson asked President George H.W. Bush for federal troops to help quell the Los Angeles riots. McHale said the Pentagon was not seeking broader powers for active-duty troops in law enforcement roles, and he warned against disregarding "200 years of history within the United States" by relying on active-duty military forces for civil order. But with 21st century threats such as chemical and biological attacks on U.S. cities, McHale said, Congress and federal agencies must establish clear guidelines on what would trigger a broad U.S. military response to restore law and order.

"It may be that the very same rationale that supports the use of the military under the Insurrection Act can be applied to catastrophic events," he said.

McHale generally praised the Pentagon's response to Katrina, including military planners' ability to dispatch 22,000 active-duty troops to the Gulf Coast within days of landfall. But poor communication between military and local authorities often hampered the effort, he said, and a lack of planning forced military officers to make too many "ad hoc" decisions.

Overall, he said, the military should have learned more from past disaster missions, such as Hurricane Andrew in 1992. "Early situational awareness was poor, a problem that should have been corrected following identical damage assessment challenges during Hurricane Andrew," McHale said. Also on Wednesday, the Pentagon deactivated Joint Task Force Katrina, the military command set up Sept. 1 to respond to the Aug. 29 hurricane.

Army Lt. Gen. Russel L. Honore, the general who led the task force, was sent back to Ft. Gillem, Ga., to resume his duties.