The Pentagon has quietly positioned air, land and sea forces in and around Boston in an unprecedented level of military support for a national political convention, a military spokesman said Monday.

"We have provided security for past conventions but nothing to the extent we have now," said Michael Kucharek (search), a spokesman for U.S. Northern Command (search), which was created after the Sept. 11, 2001, terrorist attacks to ensure that the military is prepared for security threats inside U.S. borders.

"This is somewhat of a precedent-setting situation," given that the Democratic National Convention is the first political convention since the terrorist attacks on New York and Washington, he said.

Most of the security visible on the streets of Boston is state and local police. Kucharek said the military's main role is behind-the-scenes coordination and preparation to respond to a major emergency.

Kucharek said he could not disclose numbers or other details of the military presence in Boston. He said it includes representatives of the Army, Navy, Air Force, Marine Corps and Coast Guard — both active duty and National Guard.

The Air Force is flying combat air patrols in the vicinity of Boston, he said, as is normal for what the military calls a "special security event," like the Super Bowl held in Houston last winter.

In addition to active duty forces, the Massachusetts and Maine National Guards make up "Joint Task Force-Democratic National Convention," Kucharek said. It is commanded by Army Brig. Gen. Gary A. Pappas, who is commander of the Massachusetts Army National Guard.

Kucharek said a similar arrangement would be made for the Republican National Convention in New York in late August.

By federal law the U.S. military is barred from performing domestic law enforcement duties, but Kucharek said the arrangement made for Boston this week enables the military to provide crisis support if requested by the U.S. Secret Service. He offered no specifics, but typically that means the military might be called upon to help mitigate the effects of, say, a chemical or biological weapons attack through its extensive decontamination and medical evacuation resources.

The Coast Guard has said it planned to use infrared and night-vision cameras in Boston Harbor and to randomly board commercial ships for security checks.