A Boeing passenger jet was forced to make an emergency landing after a crack in its cockpit window was discovered mid-flight.

All Nippon Airways (ANA) domestic flight in Japan, which had 65 people on board, was forced to turn back to its departure airport in Sapporo on Saturday.

It was the second safety incident to rock Boeing in as many weeks and will further deepen concerns about the reliability of its aircraft. No less than US$14 billion (NZ$22.5b) was wiped off the company’s stock market valuation in a single day after a door was torn off one of its planes being operated by Alaska Airlines on Jan 5.

More than 170 of the 737 Max 9 models have since been grounded in the United States.

The jet was about to cross the Tsugaru Strait when it turned back to Sapporo, where it landed at 12.10pm local time.

“The crack was not something that affected the flight’s control or pressurisation,” said a spokesman for ANA, Japan’s largest airline.

They added that the crack was on the outermost of four layers of glass in the cockpit window. There were no injuries or deaths and alternative flights were arranged for passengers.

It came after the Federal Aviation Administration said on Friday that all Boeing 737 Max 9 planes in the US will be grounded indefinitely “for the safety of American travellers” following the mid-flight blowout ten days ago.

“Our only concern is the safety of American travellers and the Boeing 737-9 MAX will not return to the skies until we are entirely satisfied it is safe,” FAA administrator Mike Whitaker said.

It added that it would also tighten its oversight of Boeing itself.

The company’s 737 Max jet was previously grounded for 20 months after two deadly crashes in Indonesia and Ethiopia in 2018 and 2019 respectively left nearly 400 people dead.