The crew of a Cathay Pacific flight saw what it believes was North Korea's latest missile test last week, the second airline to report sighting it.

Cathay said on Monday that the flight from San Francisco to Hong Kong reported witnessing the apparent re-entry of the intercontinental ballistic missile that North Korea launched before dawn on Wednesday.

The missile was far from the plane and operation was unaffected, Cathay said, adding that it had informed other carriers and relevant authorities.

"At the moment, no-one is changing any routes or operating parameters," the Hong Kong-based airline said in a statement.

"We remain alert and [will] review the situation as it evolves."

The North Korean missile was fired very high up, reaching an altitude of 4,475 kilometres before falling back into the Sea of Japan about 950 kilometres from where it was launched.

It was in the air for 53 minutes.

The Hwasong-15 ICBM demonstrated a greater range than other missiles North Korea has tested.

One expert estimated its range at more than 13,000 kilometres if launched on a standard trajectory, which would put Washington DC within reach.

Korean Air pilots on two different planes also reported seeing flashes of light believed to be the North Korean missile when they were flying over Japan, airline spokesman Cho Hyun-mook said.

The flights, one from San Francisco and the other from Los Angeles, were both headed for Incheon, the main airport serving Seoul, South Korea.

Experts say the chance of a missile test hitting a civilian airliner is very low.

North Korea likely chooses splashdown points for each stage that avoid airline routes, said Vipin Narang, an expert on nuclear strategy at the Massachusetts Institute of Technology.

"The problem is if the test goes awry in any way, and debris starts falling in … more heavily trafficked areas," he said in an email.

"That would still be low probability, but more risky."

AP