Washington: A top US intelligence official warned Thursday that home-grown "violent extremists" continue to pose the most likely threat to homeland security, Xinhua news agency reported.

Director of National Intelligence James Clapper told a Senate Armed Services Committee hearing that there is "serious" threat posed by extremist groups' calling for lone-wolf type of attacks against the US and other Western countries. "Of the 13 attacks in the West since last May, 12 were conducted by individual extremists," said Clapper.