Cathay Pacific said a flight from Hong Kong to New York on Saturday had to be diverted to Tokyo because a passenger felt unwell.

CX840 landed at Tokyo's Haneda Airport at 1.44am on Sunday local time and the passenger was sent to hospital.

The airline said the plane had to stay overnight in the Japanese capital due to the Civil Aviation Department's rules on how long flight crews can work.

The flight is expected to head to New York on Sunday night.

In a statement, Cathay said it provided help to other passengers and arranged hotel accommodation.

"The safety of our customers and crew members guides every decision we make. We sincerely apologise to the affected customers for the inconvenience and appreciate their understanding," the statement read.