WASHINGTON — The U.S. military expects to have 20,000 uniformed troops inside the United States by 2011 trained to help state and local officials respond to a nuclear terrorist attack or other domestic catastrophe, according to Pentagon officials.

The long-planned shift in the Defense Department’s role in homeland security was recently backed with funding and troop commitments after years of prodding by Congress and outside experts, defense analysts said.

There are critics of the change, in the military and among civil-liberties groups and libertarians who express concern that the new homeland emphasis threatens to strain the military and possibly undermine the Posse Comitatus Act, a 130-year-old federal law restricting the military’s role in domestic law enforcement.

But the Bush administration and some in Congress have pushed for a heightened homeland military role since the middle of this decade, saying the greatest domestic threat is terrorists exploiting the proliferation of weapons of mass destruction.

Before the terrorist attacks of Sept. 11, 2001, dedicating 20,000 troops to domestic response — a nearly sevenfold increase in five years — “would have been extraordinary to the point of unbelievable,” Paul McHale, assistant defense secretary for homeland defense, said in remarks last month at the Center for Strategic and International Studies. But the realization that civilian authorities may be overwhelmed in a catastrophe prompted “a fundamental change in military culture,” he said.

The Pentagon’s plan calls for three rapid-reaction forces to be ready for emergency response by September 2011. The first 4,700-person unit, built around an active-duty combat brigade based at Fort Stewart, Ga., was available as of Oct. 1, said Gen. Victor Renuart, commander of the U.S. Northern Command.

If funding continues, two more teams will join nearly 80 smaller National Guard and reserve units made up of about 6,000 troops in supporting local and state officials. All would be trained to respond to a domestic chemical, biological, radiological, nuclear or high-yield explosive attack.

Last week, Defense Secretary Robert Gates ordered defense officials to review whether the military, Guard and reserves can respond adequately to domestic disasters. Gates gave commanders 25 days to propose changes and cost estimates. He cited the work of a congressionally chartered commission, which concluded in January that the forces are not ready and that they lack equipment and training.

Bert Tussing, director of homeland defense and security issues at the U.S. Army War College’s Center for Strategic Leadership, said the new Pentagon approach “breaks the mold” by assigning an active-duty combat brigade to the Northern Command for the first time. Until now, the military required the command to rely on troops requested from other sources.

“This is a genuine recognition that this (job) isn’t something that you want to have a pickup team responsible for,” said Tussing, who has assessed the military’s homeland-security strategies.

The American Civil Liberties Union and the libertarian Cato Institute are troubled by what they consider an expansion of executive authority.

Domestic emergency deployment may be “just the first example of a series of expansions in presidential and military authority,” or even an increase in domestic surveillance, said Anna Christensen of the ACLU’s National Security Project. And Cato vice president Gene Healy warned of “a creeping militarization” of homeland security.

“There’s a notion that whenever there’s an important problem, that the thing to do is to call in the boys in green,” Healy said, “and that’s at odds with our long-standing tradition of being wary of the use of standing armies to keep the peace.”