SEOUL (THE KOREA HERALD/ASIA NEWS NETWORK) - During the Korean War, the US Air Force and Navy carried out a massive bombardment campaign against North Korea's capital Pyongyang that lasted for almost three years.

By the end of the war, when an armistice was signed in 1953, the city was virtually flattened with about 75 per cent of it destroyed.

Since then, the communist country has made efforts to build one of the world's most dense air defence networks. A multilayered air defence structure has been constructed around Pyongyang, the North's coastal corridors and its cross-border regions.

But the North's air defence capability was recently thrown into question when its radar appeared to fail to detect US supersonic B-1B bombers that conducted a flyover farthest north of the Demilitarised Zone (DMZ) than any US aircraft has flown off the North's coast this century.

"To put it simply, there were no reactions at all from North Korea when the B-1B bombers entered the area," said Mr Lee Chul Woo of the main opposition Liberty Party of Korea, who was briefed on the matter by Seoul's spy agency National Intelligence Service on Tuesday (Sept 26).

"It seems that North Korea wasn't able to take any measures because it didn't anticipate the flyover, which took place around midnight, and its radar failed to capture a strong signal," Mr Lee added after the closed-door briefing.

Theoretically, North Korea's early-warning radar is capable enough to capture incoming aircraft approaching its territory. With a range of up to 600km, the P-14 Tall King radar can detect aircraft when they cross the area between Jeju Island and the Japanese main island of Kyushu.

The two B-1B lancers - escorted by F-15 fighters - reportedly conducted aerial manoeuvres about 300km to 350km away from North Korea's port city of Wonsan, where the P-14 Tall King radar is deployed near North Korea's long-range SA-5 surface-to-air missile system.

Some analysts attributed the failure to Pyongyang's lack of power to operate the radar around the clock, while others suggested that the North could have activated the system, but its air defence officials might not have been up to the task for the surprise manoeuvre.

"Given the seriousness of the current security situation, I don't think they have turned off the radar. If this is their first failure, the air control officers' tardiness should be blamed," said a former air control officer in the Air Force, who declined to reveal his identity due to the sensitivity of the issue.

Analyst Lee Il Woo at Korea Defence Network, a Seoul-based security think tank, said the North could have failed to activate the radar system in time for the flyover, which took place at midnight without advance notice.

Even if the radars were to detect the aircrafts, it is not clear whether the North's air defence system could shoot them down, given the US modern aircraft are able to operate beyond the range of ground-to-air missiles, analysts said.

North Korea has a diverse platform of surface-to-air missiles, with the SA-3 providing short range against low-flying aircrafts, while the SA-2 provides medium-range point defence for cities and military airfields. The long-range SA-5 offers North Korea long-range, high-altitude defence.

Of these Soviet-era missiles, the SA-5 boasts the longest range of 250km, but it still fell short of intercepting the B-1B bombers, which flew about 300km away from North Korea's east coast. Most SA-5 battalions are located near the DMZ and are extended north to cover Pyongyang.

The other option for North Korea to shoot down the US fighter planes would be to scramble the North's fighter jets, but the chances that these Soviet-era war planes could put up a dogfight with the US fourth-generation fighter jets are extremely low, said former vice-chairman Shin Won Sik of South Korea's Joint Chiefs of Staff.

"During the Gulf War and Iraq War, the Iraqi air force was virtually crushed by the US without any significant air battles. The North's current air force and its air defence capability is about 10 times weaker than that of the Iraqis," he said.

According to South Korea's 2016 Defence Ministry white paper, North Korea has roughly 810 fighter planes, but experts said most of them are outdated Soviet-era aircraft and have been plagued with malfunctions.

Some analysts, however, warned against underestimation of the North's capability, saying while the North's Soviet-era air defence system and jet fighters are no match for US modern aircraft, they could still threaten non-stealth bombers - such as the B-1B.

In 1969, North Korea MiG-21 aircraft shot down a US Navy EC-121 aircraft on a reconnaissance mission over international waters, killing 30 sailors and one marine. In 1994, a US helicopter was downed by the North's portable missile, killing one pilot, after it crossed into the North's airspace.

"North Korea can shoot down anything that's not in stealth mode," said Mr Van Jackson, a former strategist at the US Defence Department, who now serves as a lecturer at Victoria University of Wellington in New Zealand.

"North Korea has very capable air defences. There's a myth out in the media right now that North Korean air defences are somehow not good enough to get US bombers and that's patently false."