US National Intelligence Director James Clapper on Thursday warned that America must be ready for a "large, Armageddon-scale" cyber attack, in remarks made at an annual conference of members of the US intelligence community in Washington DC.

"Cyber threats to US national and economic security are increasing in frequency, scale, sophistication and the severity of impact,” Clapper warned. “Although we must be prepared for a large, Armageddon-scale strike that would debilitate the entire US infrastructure, it’s not our belief that that’s the most likely scenario."

Instead of a large-scale strike, Clapper said the "primary concern is the low- to moderate-level cyber attacks from a variety of sources which will continue and probably expand. This imposes increasing costs, as you indicate, to our businesses, to US economic competitiveness and to national security."

The US government has come under a volley of cyber attacks, including a Russian hack of the Pentagon revealed in August, as well as a massive Russian breach of the State Department and White House computer systems earlier this year.

In a sign of the rising concern over the trend of cyber warfare in the private sector as well, Microsoft just this Tuesday announced its purchase of an Israeli cybersecurity startup named Adallom.

Clapper also spoke on Thursday about the crisis of Syrian refugees fleeing into Europe from the internecine civil war ravaging their country.

"As they descend on Europe, one of the obvious issues that we worry about, and in turn as we bring refugees into this country, is exactly what’s their background? We don’t obviously put it past the likes of ISIL to infiltrate operatives among these refugees," he said, using an alternate acronym for the jihadist group Islamic State (ISIS).

There is a "huge concern" over the infiltration of terrorists as Western states are pressured to accept refugees, added Clapper. Screening of the 2,000 Syrian refugees already let into the US is "pretty aggressive" according to the intelligence chief, but he said he has reservations concerning the European screening systems.