A North Korean ballistic missile was seen by the crew of several airliners last week, aviation authorities said Wednesday, raising concerns about the threat to civilian flights posed by Pyongyang's unannounced tests.

The North has conducted a flurry of missile tests this year in defiance of repeated international protests, including from aviation authorities.

Pyongyang fired an intercontinental ballistic missile (ICBM) last week, which reached an altitude of 4,475 kilometers (2,780 miles) before splashing into the sea 950 kilometers (590 miles) east of its launch site, North Korean state media said.

"A Korean Air jet flying to Incheon from San Francisco reported to Japanese controllers that its flight crew saw a flash from what was believed to be the North Korean missile," a Korean Air spokesman told AFP.

Four minutes later another Korean Air plane on a Los Angeles-Incheon flight also reported the same sighting to Japanese control, he said.

A South Korean transportation ministry official, who wanted to remain unnamed, said the flight paths of both Korean airlines were some 220 kilometers (140 miles) away from where the missile landed.

"In the clean dark sky, you can see flashes from a missile from that long distance," he told AFP.

'A bright flame falling down'

Japan's transport ministry also said air controllers in the country received reports of four such sightings.

A Japan Airlines spokesman said the cockpit crew of one of its airliners, flying from Tokyo to London, "saw a bright flame falling down" over the sea of Japan.

The reports from South Korea and Japan come after Hong Kong carrier Cathay Pacific said Monday that the crew of its flight from San Francisco to Hong Kong saw "what is suspected to be the re-entry" of the North Korean missile.

Cathay said there was no current plan to change air routes, saying its plane was "far from the event location".

In a message shared with staff, Cathay general manager Mark Hoey said the crew had described seeing the missile "blow up and fall apart", the South China Morning Post reported.

David C. Wright, a senior scientist at the Union of Concerned Scientists, wrote in a report Tuesday that the Cathay crew most likely had seen the missile's first stage burn out and fall back to earth.

"Ignition of the second stage rocket engine and separation of the first stage may have looked like an explosion that caused the missile to fall apart," he wrote.

Wright also said the "flash" seen by Korean pilots about an hour after the missile's launch would be consistent with the warhead heating up during reentry, since the missile flew for 53 to 54 minutes.

North Korea stopped giving advance notice of its missile tests in 2014.

The UN's International Civil Aviation Organization (ICAO) in October condemned the North's continued unannounced missile launches, urging it to comply with international aviation standards to prevent risks.

South Korean authorities said they warned airlines of a possible missile test a day before the launch based on intelligence reports, the transportation ministry official said.