It's unclear if North Korea could shoot down U.S. jets with surface-to-air missiles.

Highlights US Defence estimates say North Korea has 1,300 soviet-era planes

North Korea's fighter jets are unable to refuel in mid-air

North Korea's inferior fleet means reliance is on surface-to-air missiles

It's been almost half a century since North Korea shot down a U.S. military aircraft. These days its generals might find it even tougher.North Korea's top diplomat on Monday called President Donald Trump's warnings to Pyongyang a declaration of war, and said his country would be within its rights to shoot down U.S. warplanes in international airspace. Two days earlier, the U.S. had sent fighter jets and bombers the farthest north of the demilitarized zone that any had flown off North Korea's coast this century.The U.S. decided to publicize the show of force because North Korea's radars failed to spot the aircraft, Yonhap News reported, citing Lee Cheol-woo, the head of the intelligence committee of South Korea's parliament.Kim Jong Un's military could hit U.S. fighters either with its warplanes or surface-to-air missiles. In 1969, North Korean MiG jet fighters shot down an unarmed U.S. EC-121 reconnaissance plane over the Sea of Japan, about 90 miles (145 kilometers) off its coast, killing 31 people.Either option poses problems, according to Park Dae-kwang, a research fellow who studies North Korea's air force capabilities at the Korea Institute for Defense Analyses. One major obstacle is the inability of North Korea's fighter jets to refuel in mid air, he said."It's not easy to go beyond its airspace and shoot down the enemy's plane without having such an in-air fuel provision system, and I don't think the North has that yet," Park said.The U.S. Defense Department estimated in 2015 that North Korea has 1,300 planes, mostly Soviet-era models. The "technological inferiority" of its fleet means that the country's air defenses are mostly provided by surface-to-air missiles and anti-aircraft artillery, the report said.It's also unclear if North Korea could shoot down U.S. jets with surface-to-air missiles.

"Once the U.S. jet is detected by the North, not only the bomber but its escort jets are also alerted of the detection," Park said. "The bomber can conduct its mission in a stable manner while its escort jets protect it from intercept missiles."