The crew of a Cathay Pacific flight over Japan saw a North Korean ballistic missile test last week, the airline said.

The airline told the BBC that the crew flying from San Francisco to Hong Kong saw “what is suspected to be the re-entry" of a North Korean nuclear missile during a test.

Last week, the North Korean regime tested an intercontinental ballistic missile (ICBM) – the first such test in over two months – that could potentially reach any city within the U.S. The regime praised the missiles as its “most powerful”.

The details of the test remain unclear, with a U.S. official saying the missile did not manage to make a re-entry into the earth atmosphere – the key problem of the rogue communist country’s nuclear program.

Without perfecting the “re-entry” technology, such missiles pose little danger as they fall apart before re-entering the Earth.

The continued nuclear weapon tests and hostile rhetoric have increased tensions in the region, with key U.S. allies preparing for the possibility of all-out war.

The U.S. and South Korea began Monday a massive combined air force exercise in the wake of the threats from the North. A five-day drill – called Vigilant Ace –involves hundreds of aircraft and two dozen stealth jets.

South Korea’s defense ministry said the practice is aimed to improve the allies’ wartime capabilities and preparedness.

North Korean state media condemned the exercise, saying the U.S. was “begging for nuclear war” and will push the region “to the brink of nuclear war,” the BBC reported. It also said it will “seriously consider” a counter-response to the military drill.

The Associated Press contributed to this report.