North Korea on Friday ridiculed suggestions that it had sought to upstage the visit of the "so-called pope" to South Korea by firing a series of short-range rockets as the pontiff flew into Seoul.

A senior rocket scientist, Kim In-Yong, was quoted by the North's official KCNA news agency, accusing Seoul of seeking to tarnish Pyongyang's image.

South Korea "is making crazy accusations to link our strategic rocket test with the visit to South Korea by the so-called pope", Kim said.

The North fired five short-range rockets on Thursday as Pope Francis arrived in Seoul on the first papal visit to Asia in 15 years.

A South Korean foreign ministry spokesman said the timing of the launches was "quite unseemly", while local media said it was clearly aimed at upstaging the visit.

Pyongyang, however, insisted that the tests were to mark the August 15 anniversary of Korea's independence from the Japanese occupation in 1945.

Kim went so far as to suggest that any accusation of upstaging should be levelled at the pope.

"We are just curious why the pope, among all other days of this year, chose the day of our long-planned rocket launch to visit the South," he said.

The North had made no advance announcement of its rocket tests.

"We have no idea why he visits the South and have absolutely no interest in whatever plots he plans to discuss with the South," Kim added.

In an earlier despatch, KCNA said North Korean leader Kim Jong-Un had personally supervised the testing of what it called an "ultra-precision high-performance tactical rocket".

Pyongyang had trumpeted the test of a missile with a similar description in June, calling it a "cutting-edge" weapon that marked a breakthrough in national defence capability.

North Korea is not known to have a tactical guided missile, but analysis of a recent propaganda film suggested it may have acquired a variant of a Russian cruise missile, the KH-35.

The United States denounced Thursday's launches and said it was studying whether they violated UN Security Council resolutions.

"We continue to call on North Korea to refrain from undertaking such provocative actions," State Department spokeswoman Marie Harf told reporters in Washington.

Harf also voiced concern that North Korea did not give notice to alert passing ships and aircraft.

North Korea has carried out an extended series of missile tests into the East Sea in recent months, despite UN resolutions barring it from any launches using ballistic missile technology.

The North has defended the tests as a legitimate exercise in self-defence and a response to war manoeuvres involving the US and the South.