US technology companies have become "the command and control networks of choice" for extremists, the new head of GCHQ has claimed.

Writing in the Financial Times, Robert Hannigan says some US tech companies are "in denial" about how their services are being misused.

He also said UK security agencies needed support from "the largest US tech companies which dominate the web".

Extremist groups in Syria and Iraq had "embraced the web", he added.

Mr Hannigan argues that the big internet firms must work more closely with the intelligence services, warning that "privacy has never been an absolute right".

"However much they may dislike it, [US technology companies] have become the command and control networks of choice for terrorists and criminals, who find their services as transformational as the rest of us," he writes.

"The challenge to governments and their intelligence agencies is huge - and it can only be met with greater co-operation from technology companies.

"GCHQ and its sister agencies, MI5 and the Secret Intelligence Service, cannot tackle these challenges at scale without greater support from the private sector, including the largest US technology companies which dominate the web."

The debate about whether security agencies should be allowed to access personal data through social-networking sites like Google and Facebook was brought to the fore in 2013 after Edward Snowden leaked details of alleged internet and phone surveillance by US intelligence.

Mr Snowden, who has been granted temporary asylum in Russia, faces espionage charges over his actions.

(BBC)