The Pentagon's role in homeland security will be expanded with 20,000 uniformed troops expected inside the United States by 2011 to help state and local officials respond to a domestic catastrophe, FOX News has confirmed.

Under the phased plan, three rapid reaction forces will be ready for emergency response by September 2011. All the forces would be trained to respond to a chemical, biological, radiological, nuclear or high-yield explosive attack.

A spokesman for the U.S. Northern Command told FOX News that the first brigade under the phased plan will be made up of active duty soldiers; the next two will be comprised of a mix of soldiers from the National Guard and Reserves.

The Washington Post reported that 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.

Critics of the change fear 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, the newspaper said.

Click here to read the Washington Post story.