Swiss International Air Lines said Friday it had turned back a flight in mid-air heading from Zurich to Beirut because the security situation in Lebanon had become "difficult to assess".

The flight, with 138 passengers and five crew on board, did not have permission to land back in Zurich overnight Thursday so was instead diverted to Vienna, SWISS said.

"Given the latest developments in the Middle East... the situation in Lebanon last night was difficult to assess," a spokesman told AFP.

"To gain time and to be able to properly assess the situation, they decided to take this step out of an abundance of caution."

In Vienna, passengers were rebooked onto other flights.

"We hope for the understanding of the passengers concerned and did our utmost to get them to their destination as quickly as possible," the spokesman said.

Following a review, SWISS decided to maintain its flight program to Beirut and Tel Aviv, but said it would monitor developments "very closely".

The airline operates flights twice weekly from Zurich to Beirut.

Israel and Lebanon's Iran-backed Hezbollah have exchanged near-daily cross-border fire since the Palestinian militant group Hamas carried out an unprecedented attack on southern Israel on October 7, triggering war in Gaza.

On Wednesday, the Tages-Anzeiger newspaper reported that SWISS pilots and flight attendants were concerned about the situation in Israel and more and more were withdrawing from flights going to Tel Aviv.

Since the start of March, the airline has been running one return flight a day between Zurich and Tel Aviv, but plans to return to two flights daily from May 5.