The US military is under threat in its own country as homegrown Islamic extremists, including "radicalized troops," are treating military installations here as prime targets, US officials warned Congress Wednesday.

The only deadly terror strikes on US soil since those of September 11, 2001 have been against the military, with three separate attacks that left 17 people dead, most of them soldiers, according to a report released Wednesday at the first joint House-Senate hearing on homegrown terrorism.

US military installations since 9/11 have been the target of at least 33 "threats, plots and strikes," more than half of the 54 homegrown jihadist plots and attacks that have occurred or been uncovered over the past decade, the report said.

"A particularly insidious aspect of the homegrown terror threat remains radicalized troops who target their fellow brothers and sisters in arms, without regard to their faith," it said.

Republican Representative Peter King, who as head of the House Homeland Security Committee has chaired a series of hearings this year on Islamist extremism threat, lamented the "growing security threat from radicalization both internally within the military, as well as externally toward military personnel and their families residing in the United States."

The threat by radicalized members of the US armed forces "is persistent and enduring," he said in an opening statement to the hearing.

Paul Stockton, the assistant secretary of defense for homeland defense, pointed to the recent "increased numbers of American citizens or residents inspired by Al-Qaeda's ideology -- and the Department of Defense has become their target of choice."

"The primary threat to (homeland) security comes from Al-Qaeda and its affiliates," he added.

Independent Senator Joseph Lieberman said "the enemy is not a vague catchall of violent extremism, but a specific violent Islamist extremism, an exploitation and corruption, I would say, of the religion of Islam" that leaves military personnel and their families as direct targets in the United States.

Some attackers are "lone wolves" independent of any terrorist organization, while others "become radicalized in the army," said Lieutenant Colonel Reid Sawyer, director of the Combating Terrorism Center at West Point military academy.

The US Army has emerged as a preferred target for Al-Qaeda inspired individuals in the United States, in part because "the military presents a qualitatively different target when attacked at home than when engaged in combat abroad," he said.

In November 2009 a US Army psychiatrist went on a shooting rampage at a military base in Fort Hood, Texas, killing 13 people and wounding 32 others in what King has described as the worst homegrown terror attack in the United States since 9/11.

The suspect, Major Nidal Hasan, is being investigated for links to Islamic extremism, including his contacts with a radical cleric who blessed the killing spree.