London, Jan 13 (IANS): 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, BBC reported.

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

The incident was not serious but if left unfixed could have been "pretty dangerous", an aviation expert said.

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 p.m. local time, the BBC reported.

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, BBC reported.

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

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", Strickland said.

All Boeing 737-9 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, BBC reported.

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