Washington, Feb 27 (IANS): 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.

"Lone actors or groups who act autonomously will likely gravitate to simpler plots that do not require advanced skills, outside training or communication with others," he added.

Previously, secretary of Homeland Security Jeh Johnson said Sunday that extremist groups were relying more and more on independent actors to become inspired and attack on their own.

Johnson's comments came after the Somali-based al-Shabaab militant group released a video calling for attacks on shopping malls in the US, Canada and Britain.

During Thursday's hearing, Clapper said more than 3,400 Western fighters had gone to Syria and Iraq, among whom hundreds have returned home to Europe.

About 180 Americans were also involved in various stages of travel to Syria, Clapper said.