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A domestic All Nippon Airways (ANA) flight in Japan has returned to its departure airport after a crack was reported on the cockpit window during the flight.

The crack appeared in the outermost of the four layers of window surrounding the cockpit.

There were no injuries to anyone on board.

ANA flight NH1182 was flying from the city of Sapporo in Hokkaido to Toyama, on Japan's main island, Honshu.

The plane, a Boeing 737, landed back at Sapporo's New Chitose airport at around 12:10 local time (3:10 GMT).

A spokesperson for ANA, Japan's largest carrier, said that the crack had been found as the flight passed over Hakodate.

"The crack was not something that affected the flight's control or pressurisation," the spokesperson said.

The plane headed back to the airport where a safe landing was carried out.

Aviation expert John Strickland said the cause of the crack was still unknown.

"These things do sometimes happen, something may have struck the window, for example a bird, a large hailstone, it's not unheard of".

"You might occasionally get a stress fracture too, from wear and tear," he added, "but that's very rare".

He said the airline would likely have to replace the whole window, not just the broken layer, to make sure the aircraft was completely safe.

"These things do happen, it's impossible to quantify how frequently".

There were 59 passengers and six crew members on board. Alternative flights were arranged for the passengers.

This is the second incident involving a Boeing 737 model aircraft in as many weeks. However the ANA flight was not one of Boeing's 737 Max 9 airplanes, but an earlier version which was "by no means old", Mr Strickland said.

All Boeing 737 Max 9 aircraft have been grounded by the US aviation regulator the FAA after an Alaska Airlines plane suffered a blowout that saw a cabin panel fall off a new plane in mid air, leaving a gaping hole in the side of its fuselage last week.

That plane, carrying 177 passengers and crew, had to make an emergency landing in the US state of Oregon.

On Friday the FAA extended the grounding of Boeing 737 Max 9 planes indefinitely for further safety checks and said it would tighten oversight of Boeing itself.

The FAA said the 171 planes with the same configuration as the one in the incident had to stay grounded "for the safety of American travellers".