North Korea on Thursday threatened strikes on US military bases in Japan and Guam, escalating tensions as suspicion deepened that Pyongyang was behind a cyber attack on South Korean broadcasters and banks.

The tone of the strike threat, attributed to a spokesman for the army's supreme command, blended with the torrent of warlike rhetoric from Pyongyang in recent weeks, but stood out for its precise naming of targets.

Military tensions on the Korean peninsula are at their highest since 2010, with Pyongyang irate at the use of nuclear-capable US B-52 bombers and nuclear-powered submarines in joint military drills with South Korea.

"The US should not forget that the Andersen base on Guam where B-52s take off and naval bases on the Japan mainland and Okinawa where nuclear-powered submarines are launched, are all within the range of our precision target assets," the army spokesman said.

North Korea has successfully tested medium-range missiles that can reach Japan, but has no proven long-range missile capability that would allow it to hit targets on the US mainland or Guam -- more than 2,000 miles (3,200 kilometres) away.

Nevertheless, US Defense Secretary Chuck Hagel announced last week that Washington had decided to bolster missile defences along the US west coast so as to "stay ahead of the threat" from the North Korean regime.

North Korean leader Kim Jong-Un had issued a more general threat to destroy US bases "in the operational theatre of the Pacific" on Wednesday, as he directed a drone strike exercise.

Still photographs broadcast on state television seemed to show what looked like a rudimentary drone being flown into a mountainside target and exploding.

South Korea's Yonhap news agency reported last year that the North was developing unmanned strike aircraft using old US target drones imported from the Middle East.

Kim has personally overseen a host of rocket and artillery drills in recent weeks, mostly at frontline bases near the disputed maritime border with South Korea which has been a flashpoint for clashes in the past.

Since the UN Security Council tightened sanctions on North Korea over its nuclear test last month, Pyongyang has issued a range of apocalyptic threats including "pre-emptive" nuclear strikes.

It also announced it was scrapping the armistice that ended the 1950-53 Korean War, but its rhetoric has yet to be matched by any overt military action.

In Seoul, analysts said the latest threat to US bases was another attention-grabbing move.

"This is just more bluster," said Baek Seung-Joo, who heads the North Korea Research Team at the Korea Institute for Defence Analyses.

"It really seems aimed at shifting responsibility for the current tensions to the US and South Korea conducting their joint military exercises," Baek said.

The response from Tokyo was measured, with a foreign ministry official voicing regret at the North's "provocative action."

In a further sign of current tensions, North Korea conducted a one-hour civil defence drill on Thursday morning, sounding a national air raid alert over state radio.

In South Korea, government agencies were trying to confirm who was behind a concerted cyber attack the day before on three TV broadcasters and three banks that crippled their computer networks.

The regulatory Korea Communications Commission (KCC) said it had sourced the attack to an IP address in China, fuelling suspicions that North Korea may have been responsible.

Previous cyber attacks blamed on North Korea have also been tracked to Chinese sources, and security analysts in South Korea believe the North sends hackers to China to hone their skills and operate from there.

"The Chinese IP may trigger various assumptions," said Park Jae-Moon, the KCC director of network policy.

"At this stage, we're still making our best efforts to trace the origin of attacks, keeping all kinds of possibilities open."

South Korean intelligence reports say North Korea runs a cyber warfare unit with an estimated 3,000 elite hackers.