Nine years after the 9/11 attacks, the United States faces the threat of more home grown terror assaults, including prospects of suicide bombings and assassinations of top leaders, apart from Mumbai-style strikes, a leading research group in Washington has warned.

The fact that American citizens are engaged in suicide operations in Somalia raises the possibility that such attacks could start taking place in the United States itself, the Bipartisan Policy Centre's National Security Preparedness Group said in a 42-page report.

The report also warns of the possibility of assassinations of key leaders and US officials, and those who are perceived as insulting Islam.

"Because we rightly think of Al Qaeda and allied groups as preoccupied by inflicting mass-casualty attacks, we tend to ignore their long history of assassinating or attempting to assassinate key leaders and American officials," it said.

"Today, America faces a dynamic threat that has diversified to a broad array of attacks, from shootings to car bombs to simultaneous suicide attacks to attempted in-flight bombings of passenger aircraft, and that this is a state of affairs that, as we approach the ninth anniversary of the September 11 attacks, is cause for concern," Hoffman told reporters.

The fundamental conclusion of the report is that the threat has both diversified and become much more complex than it has been at any time since the attacks on September 11, 2001, he said.

Lee Hamilton, a vice chairman of the September 11 Commission which urged the government six years ago to be vigilant and nimble, said, "It's my perception -- and I could be wrong about this -- that the American people have kind of lost their focus on the threat, and I think one of the values of this report is that it brings the threat more into perspective -- as it is still serious and still dangerous".

"The American people just have to get a more realistic sense of what they're confronted with," he said.