Senior US officials are warning of a "spike" in the number of homegrown extremists who they say are increasingly difficult to detect.

US homeland security secretary Janet Napolitano told a Senate committee that security agencies are detecting increased activity by a more diverse set of extremist groups.

She says more American radicals are seeking terrorism training in the US or overseas and has warned that "a new and changing facet of the terrorist threat comes from homegrown terrorists, by which I mean US persons who are radicalised here and receive terrorist training either here or elsewhere."

"These elements make the terrorist threat more difficult for law enforcement and the intelligence community to detect and disrupt."

FBI director Robert Mueller told the committee that Al Qaeda is trying to recruit more US citizens and that American extremists are being drawn to Somalia.

Michael Leiter, head of the National Counterterrorism Centre, blames the spike in activity in part on more English-language terrorist propaganda on the internet.

Ms Napolitano also confirmed that terrorist 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.

- ABC/wires