"The threats come from a broader array of groups and regions. It comes from a wider variety of harder-to-detect tactics. And it is aimed at harder-to-secure places than before," she said.

At the same Senate hearing, Michael Leiter, head of the National Counterterrorism Center, described a "spike in homegrown violent extremist activity" that was enabled by extremist groups abroad using the Internet.

Napolitano also confirmed that terror groups posed a heightened threat to Western countries, including Europe, citing stepped up "activity."

Asked about warnings in Europe of a growing danger, she said: "Suffice it to say, we are all seeing increased activity by a more diverse set of groups and a more diverse set of threats."

The threat, which came mostly from Islamist networks, was "directed at the West generally," she said, adding that she would be discussing the issue with her European counterparts at a meeting next week.

Surge in attempted attacks

Highlighting a surge in attempted attacks or plots by U.S. nationals, top security officials cited the November shooting rampage last year at Fort Hood in Texas -- blamed on a Muslim army officer -- and the attempted bombing in May of New York's Time Square.

FBI director Robert Mueller said the al-Qaeda network had shifted the focus of its recruiting efforts since 2006 on US citizens or residents instead of volunteers from the Middle East or South Asia.

He said U.S. authorities were concerned about radicalized Americans travelling abroad to link up with extremist groups or gain experience in war zones, including Somalia.

"In particular, Somalia has garnered the attention of many American extremists, as at least two dozen Americans have successfully made it there to train or fight over the past few years," Mueller told lawmakers.

In trying to explain why the homegrown threat was expanding, the head of the Federal Bureau of Investigation said "it is possible more American extremists are feeling increasingly disenchanted with living in the United States or angry about US and Western foreign policy, making their decision to leave for extremist opportunities abroad all the more appealing."

Less extensive planning

The Sept. 11 attacks on New York and Washington killed about 3,000 people in 2001, prompting U.S. officials to fear that al-Qaeda was plotting other mass casualty events, possibly including the detonation of a nuclear device.

But Napolitano said the failed May 1 car bombing in New York's Times Square and the Dec. 25, 2009, attempt to bomb a Detroit-bound airliner showed a shift toward attacks that require less extensive planning and coordination.

Both attacks caught U.S. authorities off guard.

As a result, Napolitano warned that the United States faces a growing threat from roadside bombs like those used against American troops in Iraq and Afghanistan, as well as small arm attacks of the kind used to devastating effect in the 2008 Mumbai rampage that killed 166 people in India.

The list of potential targets now includes hotels, sports stadiums and other public areas as well as famous landmarks, airliners, chemical plants and ports.

"Unlike large-scale, coordinated, catastrophic attacks, executing smaller-scale attacks requires less planning and fewer pre-operational steps. Accordingly, there are fewer opportunities to detect such an attack before it occurs," Napolitano said.

The threat of home-grown terrorism has also taken on a higher profile with the March indictment of Colleen LaRose, a blond Pennsylvania woman known as "Jihad Jane" who is accused of plotting over the Internet to kill a Swedish cartoonist for depicting the Prophet Mohammed in a manner offensive to Muslims.

About two dozen Americans have been arrested on terrorism charges since 2009, according to Napolitano.

"While it is not clear if this represents an actual increase in violent radicalization ... it is nonetheless evident that over the past 12 months efforts by violent extremist groups and movements to communicate with and recruit individuals within the United States have intensified," she said in her testimony.