(Reuters) - The new director of Britain's surveillance agency GCHQ says U.S. technology companies have become "the command and control networks of choice for terrorists" and must do more to help the work of security and law enforcement agencies.

Robert Hannigan said Islamic State, more than any militant group before, was using mobile technology and social media such as Twitter Inc, Facebook Inc and WhatsApp to spread its message while hiding behind complex encryption tools, posing a greater challenge than ever to security services.

"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 U.S. technology companies which dominate the web," he wrote in an article for the Financial Times.

Privacy groups and some British lawmakers have accused GCHQ of widespread illegal monitoring of electronic communications after former U.S. intelligence contractor Edward Snowden leaked documents last year disclosing details of its activities and those of its U.S. counterpart, the National Security Agency.

Hannigan, just days into his new job, said that GCHQ needed to show that it was accountable for the data it used, but added that privacy "has never been an absolute right".

"I understand why (tech companies) have an uneasy relationship with governments. They aspire to be neutral conduits of data and to sit outside or above politics," he said.

"(But) however much they may dislike it, they 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 said, adding that some tech firms appeared to be in denial about the misuse of their technology.

Snowden's leaks, published in newspapers internationally, indicated that GCHQ and the NSA had intercepted and monitored phone, email and social media communications on a massive scale, causing global uproar.

In a speech made last month, Iain Lobban, outgoing chief of GCHQ, said only a "miniscule" percentage of global emails, texts and images were stored, viewed or listened to.

Hannigan, whose appointment was announced in April, has previously advised former prime minister Tony Blair on the peace process in Northern Ireland and sat on Britain's joint intelligence committee for many years.

Representatives at Facebook and WhatsApp were not immediately available for comment. Twitter declined to comment.