US spy chief James Clapper on Tuesday warned that America faced a growing threat of a crippling cyber attack and voiced alarm over North Korea's recent "belligerent" rhetoric.
In an annual assessment of global threats, the national intelligence director warned of an array of dangers around the world, from Pyongyang's bellicose stance to food shortages driven by extreme weather, but placed a particular emphasis on the threat posed by potential cyber attacks.
The United States faces a mounting danger from digital assaults on power grids and other infrastructure while cyber espionage threatens to undercut the American military's technological edge, Clapper said in his report to Congress.
Citing "increasing risk to US critical infrastructure," Clapper told the Senate Intelligence Committee that "unsophisticated" attacks could penetrate poorly protected computer networks for power grids or similar systems.
The threat of a large-scale digital assault that could cripple a regional power network was genuine but remained a "remote" possibility, his report said.
"We judge that there is a remote chance of a major cyber attack against US critical infrastructure systems during the next two years that would result in long-term, wide-scale disruption of services, such as a regional power outage," it said.
The report placed more importance on cyber threats than previous years and devoted more words to the problem than to Islamist militants in Afghanistan.
With tensions spiking on the Korean peninsula, Clapper also told senators he was dismayed by "very belligerent" rhetoric coming from the North's young leader Kim Jong-Un.
Asked for his view of Pyongyang's latest threats at the hearing, Clapper said he was "very concerned about what they might do."
Clapper's report said North Korea would likely only use nuclear weapons if it perceived a threat to its survival, but the United States remains uncertain how Pyongyang would define such a threat, AFP reported.
"Although we assess with low confidence that the North would only attempt to use nuclear weapons against US forces or allies to preserve the Kim (Jong-Un) regime, we do not know what would constitute, from the North's perspective, crossing that threshold," the assessment said.
The United States, however, faces a challenge trying to discern North Korea's strategic calculations when it comes to its nuclear weapons.
"We do not know Pyongyang's nuclear doctrine or employment concepts," the report said.
North Korea's Kim has threatened to "wipe out" a South Korean island amid fresh international pressure and new sanctions over the North's nuclear weapons and missile tests.
While much of the North's rhetoric has been dismissed as bluster, the latest threat to the border island of Baengnyeong, which has around 5,000 civilian residents, appears credible, analysts say.
The intelligence report also addressed the state of Iran's nuclear program, saying Tehran could not produce enough highly-enriched uranium for an atomic bomb without being detected.
While Iran has made strides in its nuclear program, "we assess Iran could not divert safeguarded material and produce a weapon-worth of WGU (weapons-grade uranium) before this activity is discovered," the report said.
Iran's declared nuclear sites are subject to monitoring from the UN's atomic watchdog, the International Atomic Energy Agency, as well as clandestine surveillance from US and other spy services.
The intelligence assessment, however, found that Iran has moved forward with its uranium enrichment efforts over the past year.
The assessment reiterated an analysis last year from intelligence agencies that Iran had not yet opted to build nuclear weapons and that the regime's policy was based on a "cost-benefit" approach.
"We do not know if Iran will eventually decide to build nuclear weapons," he said.
But because Iranian leaders put a high priority on preserving their power and would carefully weigh the risks of obtaining nuclear weapons, the United States and its allies had an opportunity to exert influence over Tehran's ultimate decision, he said.
The American intelligence community also believed Iranian leaders are not seeking a direct confrontation with the United States, as it could risk their political control, the report said.