Kim Yo Jong's defence of North Korea's satellite capabilities comes after the isolated country said it conducted an "important final-stage" test for the development of a reconnaissance satellite.

But experts in Seoul quickly raised doubts, saying the quality of the photos -- presumably taken from the satellite -- was too poor.

In a lengthy, vitriolic statement carried by the official Korean Central News Agency, Kim said it was "too inappropriate and careless" to evaluate Pyongyang's satellite development progress and capability based on the two images.

She insisted a camera installed on the satellite had the "reliability of ground control including attitude control and shooting control command in a suitable space flight environment".

Kim also said the satellite's data transmission devices and encryption processing technology were reliable.

"We carried out a necessary test and reported the significant and satisfying result, which was not lacking," she said.

The development of a military reconnaissance satellite was one of Pyongyang's key defence projects outlined by her elder brother Kim Jong Un last year.

North Korea is under biting international sanctions for its nuclear weapons programmes, but peaceful satellite launches are not subject to the same level of restrictions.

But analysts say developing such a satellite would provide North Korea with cover for testing banned intercontinental ballistic missiles (ICBM), as they share much of the same technology.

Earlier this year, Pyongyang carried out two launches, claiming it was testing components for a reconnaissance satellite, which the United States and South Korea said likely involved components of its new Hwasong-17 intercontinental ballistic missile (ICBMs).

The younger Kim rebuked claims that the North's satellite launches were thinly disguised firings of banned ICBMs.

"If we develop ICBMs, we will fire ICBMs, and not test long-range rockets disguised as satellites," she said.

Kim also dismissed analysts doubting that the North has the advanced technology needed for the rocket to survive re-entry into the Earth's atmosphere, saying she would explain it in "an easy-to-understand manner" to their naysayers.

"If the atmospheric re-entry technology was insufficient, it would not be possible to receive remote data from the pilot combat unit until the moment of impact," she said.

- Air drill -

The weekend's launch comes after a year of unprecedented weapons tests by North Korea, including the launch of its most advanced intercontinental ballistic missile the month before.

The United States and South Korea have warned for months that Pyongyang is preparing to conduct its seventh nuclear test.

The two countries held a joint air drill on Tuesday, and deployed a US B-52H strategic bomber to the Korean peninsula, the South Korean Joint Chiefs of Staff said in a statement.

The long-range heavy bomber was part of an exercise that included the US and South Korea's most advanced jets -- including the F-22 and the F-35 stealth fighters.

Experts say North Korea is particularly sensitive about US-South Korean joint air drills, as its air force is one of the weakest links in its military, lacking high-tech jets and properly trained pilots.